TL;DR - In the novel, Xiao Yao and Cang Xuan are blood-related cousins through Xiao Yao's mother and Cang Xuan's father. After Cang Xuan's parents died, Xiao Yao's mother looked after Cang Xuan (her nephew) as if he were her own child. Cang Xuan and Xiao Yao lived together for eight years after Xiao Yao's mother's divorce, and they referred to each other as 哥哥 (Ge Ge / older brother) and 妹妹 (Mei Mei / younger sister) even before Cang Xuan's parents died. 

In the drama, some details have been included to make it seem like Xiao Yao and Cang Xuan are not blood related (with Xiao Yao's mother perhaps only being the Queen of Xiyan's disciple and not her biological daughter). But some details still strongly indicate that Xiao Yao and Cang Xuan are blood-related cousins. Given the conflicting details, you can pick the interpretation you prefer.

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Feel free to use this thread to correct any of the items below if I've gotten anything wrong, or to suggest any details that should be added. 

All references to "the novel" mean Koala's english translation of the 2013 version of the novel, unless specified otherwise. Timestamps marked with "VK" are aligned with the timing of the version of the drama available through Viki. Timestamps marked with "YT" are aligned with the timing of the version of the drama available on Youtube.

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In the Novel - Summary

Xiao Yao and Cang Xuan are blood-related first cousins.

Xiao Yao's mother's official name was Xuan Yuan Ba. She went by Xiling Ba (taking her mother's family name) to wander the wilderness freely. Her nickname was Ah Heng. She was the only daughter of the Yellow Emperor of Xuan Yuan and his official Empress, Xiling Lei Zhu. Which made her the princess of Xuan Yuan. 

In addition to her daughter, Empress Lei Zhu had three sons: Qing Yang, Yun Zhe, and Chang Yi. Ah Heng referred to Qing Yang as her Eldest Brother, to Yun Zhe as her Second Brother, and to Chang Yi as her Fourth Brother. Xuan Yuan Chang Yi was Cang Xuan's father. Making Ah Heng Cang Xuan's aunt, and making Xiao Yao and Cang Xuan first cousins. Xiao Yao refers to Chang Yi as "Fourth Uncle" and to Chang Pu as "Fourth Aunt". 

In addition to his official Empress, the Yellow Emperor had three consorts and his third wife (Tong Yu) was his favourite. Eldest Prince Qing Yang was suspected / accused of killing Third Prince Hui, who was Tong Yu's son. In retaliation, ninth Prince Yi Peng, also Tong Yu's son, contributed to Fourth Prince Chang Yi's death. To avenge her husband, Chang Pu wanted to kill Yi Peng but ended up killing his mother (Tong Yu) instead before committing suicide. To prevent Yi Peng from killing Cang Xuan to avenge Tong Yu, Ah Heng killed Yi Peng (her ninth brother and childhood friend). 

Although the Yellow Emperor has many other sons (including Hui and Yi Peng), Xiao Yao and Cang Xuan are the only grandchildren of the Yellow Emperor and his official Empress.

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In the Drama - Summary

XY's mother is only referred to as Xiling Heng, A'Heng, and Princess General in the drama. Never as Xiyan Ba. And A'Heng says to CX that her last name is not Xiyan. Note that Xiling is the Queen of Xiyan's family name. A'Heng says she was taught by CX's grandmother and refers to CX's grandmother as "shifu" rather than "mother". Shifu is also the term that CX uses to refer to the King of Haoling. CX and the Qingshui stone spirit refer to XY's mother as the Queen of Xiyan's disciple. 

After divorcing the King of Haoling, XY's mother says that she is only a citizen of Xiyan. She was given the "Princess General" title by the King of Xiyan right before she left Zhaoyun Peak to lead Xiyan's army in their final battle with Chengrong's army (led by Chi Chen). All of this seems to suggest that XY's mother was not the biological daughter of the Queen of Xiyan (Xiling Xie) and the King of Xiyan.

However, XY refers to CX's parents as her aunt and uncle. CX refers to XY's mother as his aunt. They both refer to the King and Queen of Xiyan as grandfather and grandmother (or grandma). But theoretically this could still happen even if XY's mother was a disciple to the Queen of Xiyan if she was accepted into the family as an adopted daughter. Although it seems a little strange, in that case, for XY's mother to call herself Xiling Heng instead of Xiyan Heng and yet XY is considered the granddaughter of both the Queen (Xiling Xie) and the King of Xiyan. 

More confusingly, XY says that she and CX have a blood relation, and the King of Xiyan tells XY that she looks nothing like her own mother and instead her features strongly resemble the features of her grandmother (i.e., the Queen of Xiyan) back when the King of Xiyan first met her. These two details both make it seem like XY and CX must be blood relatives. In which case, all the "disciple" references might just be smoke and mirrors to help the drama pass censorship restrictions without actually changing the fact that XY and CX are blood-related first cousins. 

It's also possible that the "disciple" references are re-dubbing changes that were made at the last minute, like the change that was made to XY and CX's childhood agreement (see the difference between the scene from the trailer and the scene from the drama below) and the production team just wasn't able to fully make all the changes necessary to erase all the references to CX and XY being blood relatives. 

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Names

Cang Xuan is "Xiyan Cang Xuan" in the drama, "Xuan Yuan Cang Xuan" in the 2019 version of the novel, and "Xuan Yuan Zhuan Xu" in the 2013 version of the novel. 

Cang Xuan's father is "Xuan Yuan Chang Yi" in the novel and "Xiyan Zhongyi" in the drama.

Cang Xuan's mother is "Ruo Sui Chang Pu" in the novel and "Zhuoshan Chang Pu" in the drama. Chang Pu's family name is shown on her grave in episode 22.

Cang Xuan's grandmother is "Xiling Lei Zhu" in the novel and "Xiling Xie" in the drama. Feng Long refers to her as Lady Xiezu in episode 24 and Xing Yue refers to her the same way in episode 26.

XY's mother is "Princess Xuan Yuan Ba", "Xiling Ba", "Ah Heng" and the former Empress of Gao Xing in the novel. In the drama, she is "Xiling Heng", "A'Heng" and the former Queen of Haoling. By the King of Xiyan's decree, she is given the title "Princess General" before she leads the Xiyan army against General Chi Chen's forces.

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Timestamps from the Drama

Episode 1 (VK 2:45) - The Queen of Xiyan is introduced as Xiling Xie

Episode 1 (VK 3:19 / 17:26 / 18:23 / 19:48) - XY refers to Xiling Xie as "阿婆" (Āpó / grandma). 

Episode 1 (VK 5:31 / 17:24 / 18:12 / 19:48) - CX refers to Xiling Xie as "奶奶" (Nǎinai / grandmother). 

Episode 1 (VK 6:39) - XY's mother is introduced as Xiling Heng

Episode 1 (YT 7:50) - CX refers to Xiyan Yipeng as "Ninth Uncle". 

Episode 1 (YT 8:14) - XY refers to CX's mother as "Auntie". 

Episode 1 (YT 8:31) - CX's mother is introduced as Chang Pu. 

Episode 1 (YT 8:56) - CX's mother (Chang Pu) confirms that her husband is Xiyan Zhongyi. 

Episode 1 (YT 9:02) - Xiyan Yipeng (a son of the King of Xiyan and one of his other wives, not Xiling Xie) refers to CX's mother (Chang Pu) as "sister-in-law" and to Xiyan Zhongyi as his brother. 

Episode 1 (YT 9:42) - CX's mother (Chang Pu) confirms that she is the head of the Ruoshui Clan.

Episode 1 (YT 12:15) - CX's mother (Chang Pu) refers to XY's mother (Xiling Heng) as "little sister". Remember, in the novel Ah Heng was Chang Yi's little sister. Since Chang Pu was Chang Yi's wife, Ah Heng was also Chang Pu's little sister-in-law. 

Episode 1 (VK 16:02 / 16:29) - XY's mother (Xiling Heng) refers to Xiling Xie as "师傅" (Shīfù / master / teacher). She says she will look after CX and he will be her child from now on, just like XY.

Episode 1 (YT 16:44) - Xiling Xie said she couldn't protect her son or her daughter-in-law (i.e., Zhongyi and Chang Pu). 

Episode 1 (VK 16:52 / 28:46) - XY's mother (Xiling Heng) refers to herself as a citizen of Xiyan, and no longer the Queen of Haoling. 

Episode 1 (VK 25:55):

XH: Cang Xuan, I am indeed a woman, but I am also a solider of Xiyan, just like your father. Although Xiyan is not my last name, I've been taught by your grandmother to protect this kingdom and its people. 

...

CX: I'm also a Xiyan citizen...

Episode 1 (VK 29:27) - By the King of Xiyan's decree, XY's mother (Xiling Heng) is made the Princess General who will lead Xiyan's army into battle. 

Episode 1 (VK 31:42):

King of Xiyan: Jade Mountain is isolated. It's a simple place. The Queen Mother has strong spiritual power and she [knew] your grandmother. She will certainly raise Xiao Yao wholeheartedly. 

Episode 1 (VK 41:48) - "The Queen of Xiyan on Zhaoyun Peak [had] three sons and one disciple."

Episode 2 (YT 3:56)

QS Town Resident: The King of Haoling is the princess' father. The King of Xiyan is the princess' maternal grandfather. Neither emperor can find her?

Episode 18 (VK 28:14):

CX: A'Nian, Xiao Yao's mother is the person I admire the most. She was the disciple of Xiling Xie, the Queen of Xiyan. She was also the bravest Princess General. Shifu married her with the grandest ceremony. 

Episode 19 (VK 9:44 / YT 10:46):

CX: Xiao Yao, if... I mean if one day, between me and Tushan Jing, you could only choose one, who would you choose?

XY: Cang Xuan, I've been a man for hundreds of years. Neither my mindset nor my actions are those of a little girl. Today's intimacy between lovers may turn into heartlessness tomorrow. Love arrives passionately and fades fleetingly. But the two of us share a blood relation, as well as a life.  

Episode 21 (VK 7:10):

King of Xiyan: You don't look like your mother, but your face and mouth really resemble those of your grandmother. Exactly like hers when I met her. 

XY: I hadn't seen my grandmother when she was young, but in my memory, she was always old and withered. Not like other women who always use spiritual power to maintain their youthful beauty. 

King of Xiyan: Your grandmother was once as young as you. 

Episode 21 (VK 21:31):

King of Xiyan: So you are this kind of person: cold and indifferent. Completely different from your mother and grandmother. 

XY: What's so good about being like them? They ended up suffering while men reap the benefits. I've suffered enough in my life. I won't go looking for trouble from men. 

King of Xiyan: You're in the royal family. Thinking this way is a good thing. 

Episode 22 (YT 25:22): CX and XY bow to Xiyan Zhongyi and Zhuoshan Chang Pu's tomb. 

Episode 22 (YT 25:50): CX and XY bow to Xiling Heng's tomb, which is positioned near Xiling Xie's tomb with similar prominence to the tombs of Xiling Xie's sons (including Xiyan Zhongyi).

Episode 24 (VK 7:50):

Feng Long: It was wise of His Majesty and Lady Xiezu to build a town here. 

Episode 24 (VK 20:32)

XY: Are you worried that your uncles won't let you go?

Episode 26 (VK 19:53)

Xing Yue: The people in this garden are all my relatives. 

XY: But we aren't. 

Xing Yue: Why not? Cang Xuan's grandmother, Lady Xiezu is the lady of the Xiling family. His grandmother's mother is my grandfather's great aunt. Speaking of which, I should call him uncle. His uncle, the leader of the Xiling family, married the lady of the Jiang family. Their son and his cousin are the cousins of Miss Jiang. Miss Jiang is the cousin of Miss Shen. I'm Miss Shen's cousin, so I can call him cousin. 

XY: I'm totally confused by all these cousins on both sides. 

Xing Yue: This was enough to confuse you? The Four Great Clans have been allied with the clans in the Central Plains. Starting from Lady Xiezu, you all have relatives in the Central Plains

CX: Indeed it's true. Thinking about it carefully, we're all relatives

XY: [thinking to herself] No wonder my grandfather has always been worried about the Central Plains. All the clans' bloodlines are intertwined. 

Episode 34 (VK 34:14):

CX's uncles: The Xiling family and Cang Xuan are blood relatives

Episode 39 (VK 22:10):

King of Haoling: A'Heng, do you allow Xiao Yao to marry Chishui Feng Long? 

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Childhood Agreement Between XY and CX

From chapter 38 of the novel:

When he met Xiao Yao for the first time, they didn’t get along at all. He was a boy but he couldn’t beat the rough and tumble Xiao Yao in a fight. He even tried to be sly and find a way to get rid of her. But gradually the two of them played together and when his parents both died it was Xiao Yao who kept him company every night. When he had nightmares it was Xiao Yao who kissed him on the forehead and vowed,I will be with you forever.” He didn’t believe her. “You’ll get married eventually and leave me.” She hurriedly said, “I won’t marry anyone else, I will marry you and never leave you.”

From Five Gods Mountain to Xuan Yuan Mountain, from Xuan Yuan Mountain to Sheng Nong Mountain, Xiao Yao accompanied him each step of the way. No matter what happened, no matter what state he was in, she resolutely stood beside him. When Yu Jiang tried to assassinate him, it was Xiao Yao who used her body to shield him. When he was in the hidden chamber detoxing from the drugs, it was Xiao Yao who endured the agony with him and was willing to be injured but wouldn’t use Jin Xuan’s suggestion to tie him up. She knew that if she suggested it then he would do it…..

From chapter 26 of the novel:

Seeing [Cang Xuan] getting married, Xiao Yao plastered a smile on her face but had no happiness in her heart. They stumbled, they fell, they drifted, they fought, along the way her and [Cang Xuan] both grew up and now he was getting married. But this wedding wasn’t what Xiao Yao had envisioned.

In her memories: her Eldest Uncle’s wedding to the Sheng Nong Kingdom’s Princess was a lavish ceremony. She and [Cang Xuan] got in a fight, he said a married daughter was like water splashed outside. She remembered later her Fourth Auntie committing suicide, [Cang Xuan]’s nightly nightmares, her comfort that she would forever be with him. [Cang Xuan] said she would marry one day and leave him, she innocently said that she wouldn’t marry another, she would marry him.

In the trailer (https://youtu.be/MnJ2JHkOnPE?si=0uPx9LzKjLErvfwS&t=179):

XY: If I marry you, does it mean we will never be apart for the rest of our lives? 

CX: Yes! My father said that "husband and wife" are people who spend their lives together, never getting separated, no matter what happens

XY: Then we have a deal. When I grow up, I will marry you and be your wife!

CX: Deal!

In the drama (episode 1, VK 23:16):

XY: Anyway, I'm used to living on Zhaoyun Peak. I will stay here and keep you company forever, okay?

CX: Okay. But you will grow up. You'll settle down and start a family. Eventually you'll have to leave Zhaoyun Peak and me. 

XY: Eh. We won't ever be parted as long as I'm your little sister

CX: Alright, you're my little sister! And I'm your older brother, forever. No matter what, we'll be together forever. 

XY: Deal! Together forever. We will never be parted. 

CX: Deal!

I googled a question whether XY and CX are related by blood and it brought me here. Thank you so much for the details. I am super impressed and the novel sounds amazing. Are there English translations available? My mandarin is not too good, unless there is an audio version of the Chinese novel? I will get so slow in reading the mandarin text myself.  

 YTF123456:
googled a question whether XY and CX are related by blood and it brought me here. Thank you so much for the details

You're welcome! I'm glad it was helpful. ^^

 YTF123456:
the novel sounds amazing.

I like the novel more than I like the drama.

 YTF123456:
Are there English translations available?

There is a complete translation of the 2013 version of the novel that was done by a fan who goes by Koala. It isn't perfect (a few mistranslations have been discussed in the discussion threads here) and it doesn't reflect the changes that Tong Hua made in the 2019 version of the novel, which is closer to the drama, but it's a pretty impressive feat for a fan translation.

Link: Lost You Forever Archives - Page 7 of 7 - A Koala's Playground (koalasplayground.com) 

Koala's translation also covers a few chapters from the prequel, Once Promised. A few other fans have translated or created summaries of bits and pieces of that novel, but unfortunately no one has done a full translation yet, as far as I am aware. 

For anyone interested in knowing more about the Xuan Yuan royal family, this summary from Koala provides some helpful information:

The Yellow Emperor and his Empress Xi Ling Lei Zhu’s backstory.

Xi Ling Lei as a young girl was the only daughter of the Xi Ling clan, one of the Four Great Clans. She was world renowned because she was a master insect breeder and was able to breed silk worms that could make the most beautiful silk for clothing. She was willful and proud because she was so talented, beautiful, and came from such a powerful family background.

She ran away from home and roamed the vast wilderness with her two best friends, one would go on to become the next Flame Emperor of the Sheng Nong Kingdom, and the other would become the next Royal Mother. But when they were young they were mischievous and playful, got in brawls and generally loved life wherever they went.

One day they were traveling through Xuan Yuan and Lei saw a strapping young man and fell in love, he would become the future Yellow Emperor. She had marriage offers by the dozens and thought this young man would love her on sight. When she approached him, he turned her down and said he was already in love with another woman.

Lei left with her two best friends but could not forget him. She decided to leave her best friends and go back and win his heart at all costs. Her pride would not let her lose, plus she really could not forget him. She went back to Xuan Yuan and for the next hundred years she became his friend, confidante, and taught him everything about building up a kingdom and empire since she was so well learned. They became very close and one day she revealed who she really was – Xi Ling Lei, probably the most famous bachelorette in the vast wilderness.

He asked her to marry him and she agreed. Right before her wedding day, a pregnant woman came to her and begged her through tears not to marry her husband. She was the Yellow Emperor’s first love and carrying his child. She was not from a powerful family and was just a nobody Xuan Yuan girl. Now that she was pregnant, her father and brother would beat her for shaming the family name. If Lei wouldn’t call off the wedding, then please have the Yellow Emperor take her as a concubine officially. Lei was shocked and dismayed and coldly turned her down, she was not going to suffer the embarrassment of having her marriage involve a concubine and one who was carrying her husband’s child.

On her wedding procession day, the woman came back and she was bloody, bruised, and battered, wearing a sack cloth and carrying a bloody piece of flesh in her arms that was her aborted baby. She cursed Lei and all her future children for taking her man and killing her child. Lei was shaken but went on to marry the Yellow Emperor and for the next thousand years they built the Xuan Yuan Kingdom together, seemingly having the happiest of marriage and Lei put that woman behind her. Lei was happy and bore him two sons, eldest Qing Yang, second Yun Zhe.

Then the Yellow Emperor asked to take his first consort to stem a rebellion in the East, Qing Yang was angry but Yun Zhe understood the implications and urged his mother to agree to it. She did. Only later did she realize he was setting her up so that he could take a third consort from another small Xuan Yuan tribe. When Lei met her she was floored – the third new consort was none other than the pregnant woman who begged her to not marry that day. Her name was Tong Yu and she was the Yellow Emperor’s first love and he simply was biding his time until be could bring her back. She was pregnant already and gave birth to third prince Hui, followed shortly by the Empress going into labor early with her third son Chang Yi, who became the fourth prince of Xuan Yuan.

From then on, Consort Tong Yu had it out for Empress Lei, never forgetting her curse that she and all her children would suffer for what Lei had done to steal her man. Eldest prince Qing Yang hated politics so second prince Yun Zhe would go to war for his brother and in one battle, he died after falling in a volcano. The only person present was third prince Hui, and Qing Yang accused him of killing Yun Zhe. But the Yellow Emperor refused to investigate and closed the matter as death during battle. Fourth Prince Chang Yi’s death (Zhuan Xu’s dad) was because ninth Prince Yi Peng (another one of Consort Tong Yu’s sons) set it up so the reinforcements wouldn’t arrive on time, which was in revenge for Prince Qing Yang causing the death of his blood brother third prince Hui. You see how this is just the worst vicious cycle EVER. Ah Heng ended up killing her own ninth brother Prince Yi Peng to keep Zhuan Xu safe in the future. 

Empress Lei Zhu and Consort Tong Yu and their respective kids would be battling it out until the bitter end, all because these two women were stupid enough to fall for a man more ambitious than his love for either of them or for his kids. In the end Lei completely broke it off with her husband, believing that he never loved her and it was all to build his kingdom. In truth he did love her, as he did still love his first love. He tried to have it all but he bit off more than he can chew.

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For reference, the Yellow Emperor's children:

Name and TitleStatus in LYFMotherChildren
N/ADead / never born

Tong Yu blamed Lei Zhu for her first child's death before birth. 
Yellow Emperor's third wife, Consort Tong Yu.

Tong Yu was the Yellow Emperor's first love. She became pregnant with the Yellow Emperor's first child before she married the Yellow Emperor + before Lei Zhu married the Yellow Emperor. 
None

First Prince Qing Yang

Dead

Shao Hao, Yi Peng, the Yellow Emperor and Chi You contributed to Qing Yang's death
Yellow Emperor's first wife, Empress Lei ZhuNone
Second Prince Yun ZheDead

Hui contributed to Yun Zhe's death
Yellow Emperor's first wife, Empress Lei ZhuNone

Third Prince Hui

Dead

Tong Yu and Yi Peng believed that Qing Yang caused Hui's death
Yellow Emperor's third wife, Consort Tong Yu?
Fourth Prince Chang Yi

(Zhongyi in the drama)
Dead

Yi Peng contributed to Chang Yi's death.

Shao Hao refused to provide help to save Chang Yi.
Yellow Emperor's first wife, Empress Lei ZhuCang Xuan (Zhuan Xu in the 2013 version of the novel)
Fifth Prince Cang Lin

(Deyan in the drama)
Alive

Cang Xuan / Zhuan Xu's rival to become the heir to the Yellow Emperor's throne.

One of the people the Yellow Emperor asked to try to convince Xiang Liu to switch sides.
?Shi Ju

(Yue Liang in the drama)
Sixth Prince XiuDead

Killed in connection with a plot to assassinate the Yellow Emperor when he visited the Middle Plains before they were conquered. 
Yellow Emperor's second or fourth wife, a consort who was a daughter of the Wan Lei family?
Seventh Prince Yu YangAlive

Supports his brother Cang Lin's attempts to become the Yellow Emperor's heir.
?First son = Yu Hao

Second son = Cui Liang

(Shiran in the drama)
Eighth Prince QingAlive, but imprisoned

Locked up for life in connection with a plot to assassinate the Yellow Emperor when he visited the Middle Plains before they were conquered.

One of the people the Yellow Emperor asked to try to convince Xiang Liu to switch sides.
Yellow Emperor's second or fourth wife, a consort who was a daughter of the Wan Lei family?
Ninth Prince Yi PengDead

Killed by Ah Heng to protect Cang Xuan / Zhuan Xu.
Yellow Emperor's third wife, Consort Tong Yu?
Tenth Princess Ba (aka Ah Heng)Desert beast / deadYellow Emperor's first wife, Empress Lei ZhuJiu Yao (aka Xiao Yao)


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From Koala's translation of Chapter 22:

Zhuan Xu coldly huffed, “A load of nonsense! My Auntie did kill my Ninth Uncle, but that wasn’t because of some affair coming to light. It was….” Zhuan Xu paused, “It was because my Mom wanted to kill my Ninth Uncle and inadvertently killed my Ninth Uncle’s mom, Consort Tong Yu, who was my Grandfather’s third wife. My Mom knew my Ninth Uncle would kill me in vengeance so she begged my Auntie to protect me before she committed suicide. My Auntie agreed to protect me, and to do so she killed my Ninth Uncle.”

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xiaoyuer translated the scene where Yi Peng died in Ah Heng's arms:

Scene Translation: At Death’s Door, Their Final Meeting

That day was also Tong Yu Shi’s [1] funeral, but because Lei Zu is the Queen, Qing Yang is the future Huang Di [Yellow Emperor] in everyone’s hearts, and Huang Di announced that Chang Pu had been wounded on the battlefield, and had died from her wound, so her funeral was obviously more important than the Tong Yu Shi “passing due to illness”. Tong Yu Shi’s grave was drearily silent, with only Yi Peng accompanying her.

A’Heng walked over, Yi Peng shouted: “Get lost!”

A’Heng ignored him, and still walked toward the grave. Yi Peng flew into a rage and raised his palm to strike A’Heng, each strike was violent with the intent to kill: “Have you come to gloat?”

While evading him, A’Heng said: “What should I gloat about? Gloat that you have my caused the deaths of my three blood-related brothers? Gloat that your mother has forced my mother so much that she does not have many days to live?”

Surprised, Yi Peng uncertainly asked: “What rubbish are you talking about? Isn’t Qing Yang alive and well?”

“He is already dead. When you plotted to make our Royal Father mistakenly believe that he really wanted to poison our Royal Father, the poison he drank began to take effect just when he was dueling with Chi You, so he died at Chi You’s hands.”

“Then that Qing Yang who is living in seclusion underneath Gui Xu Shui to heal his wounds is fake?” Yi Peng began to laugh loudly, laughing so hard that he could not catch his breath: “Mother, did you hear this? The villain who caused elder brother’s death had actually died long ago! That old poisonous woman is also about to die!”

A’Heng watched coldly. After Yi Peng was finished laughing, he then looked at A’Heng and said: “With you personality, this should be the memorial you are giving to me. Little sister, how are you planning to kill me?”

A’Heng said: “I already have.”

Yi Peng smiled and said: “I believe you, but I don’t understand.”

“A few thousand years ago, my mother and Yan Di [Flame Emperor] were sworn brother and sister. When Yan Di was critically ill, he gave me his life’s work, Shen Nong’s Book of Chinese Medicine.”

Yi Peng suddenly understood: “No wonder you could hide how long you were pregnant with that little bastard, but even with Shen Nong’s Book of Chinese Medicine, you cannot easily poison me.”

“I know, but have you forgotten? We were taught by the same shifu [2], and I am very familiar with how your lingqi [3] operates. I prepared the poison in two steps. The first step, was just right here.” A’Heng looked at Tong Yu Shi’s tomb: “These past few days, you have been kneeling here, often for the entire night. When you are grieved, the lingli that protects your body weakens a lot and can easily be invaded by evil influences.”

“This is an open grave that has been blessed with lingli, if you used poison, it would have definitely changed.”

“Yes, so what I used cannot be considered a poison. Actually it is a medicine to greatly increase lingli, and can cause your lingli to rise quickly in a short amount of time. I had just told you that Qing Yang had already died, and your spirit was agitated. When you laughed uproariously, you breathed in many things that you shouldn’t have breathed in. This is also not a poison, but when paired with the medicine that was already in your body, combined with how your lingli specially operates, it will lead all your lingli to converge in your heart. In the end, your heart will not be able to control your powerful lingli, so your heart will explode and you will die.”

Yi Peng paused in surprise and A’Heng said: “I am Yan Di of Shen Nong’s disciple, not the disciple of Jiu Li’s King of Poison. I do not need poison to take someone’s life.”

Since life no longer has joys, why should he be afraid of death? Yi Peng smiled, gathered all his lingli and wanted to kill A’Heng with one strike: “Then that’s fine. We will go together!”

A’Heng quietly stood there without moving. Yi Peng released half of the force in his palm and fell down in front of the grave.

The lingli he had just gathered all surged into his heart, the blood vessels in his heart were seemingly about to burst open, putting him in so much pain that his entire body twitched.

Yi Peng tried his best to control the turbulently flowing lingli. His face turned from white to green, and from green to red, his countless lingqi seemed like a countless number of poison snakes delving into and biting at his heart. His face trembled with pain.

A’Heng squatted next to him, the emotion in her eyes was very complex. She hated him, so she planned this painful death for him, but seeing him in so much pain now, she also was in a lot of pain.

“Yi Peng, if I do not kill you, will you still try to kill Zhuan Xu?”

Yi Peng was in so much pain that his face was contorted, but he still laughed wildly and fiercely said: “Yes! His mother killed my mother. How could I let him off? All of you must die… ah!” He was in so much pain that he could no longer speak, his hands beating at his chest. He tore his clothes to shreds, revealing a scar on his right shoulder. It was circular with five faint purple shadows, in the shape of a claw.

A’Heng’s expression quickly changed and her eyes were filled with tears.

“Ah—ah—“ Yi Peng was in so much pain that he cried out pitifully, falling at A’Heng’s feet, coiling into a ball, and the scar on his shoulder became even more pronounced.

A’Heng stretched out her trembling hand, placing it on Yi Peng’s shoulder, sending her lingli into his body, slowly relieving Yi Peng’s pain. Yi Peng tore at her, pushed her, and hit her: “Scram!” But she did not move away, allowing Yi Peng to strike her. Yi Peng ripped her sleeve, revealing her arm. There was a scar on her arm, very similar to the scar on Yi Peng, and looked like half of a claw.

As Yi Peng hit her arm, he suddenly slowed.

A’Heng’s lingli began to work and the pain slowly vanished. This disappearing pain seemed to have emptied the grief and hate in his heart. His heart seemed to have turned into a deep pool of water, clean and clear, with sunlight passing through. At the bottom of the pool was a young man who did not know what worries were.

Their Royal Father mandated that he would study together with A’Heng, and chose the same shifu for them, but his mother forbade him from speaking to A’Heng. Every day in the early morning, A’Heng would always hide in the corner waiting for him, and they would go to class together, holding each other’s hands.

In the summer afternoons, they jumped into water together from a high bridge, competing who would make the largest splash. In the snowy winter, they would lie in the snow together, using wicker baskets to catch birds. He gave the parrot he liked most to A’Heng and A’Heng embroidered a hebao [4] for him, making the most beautiful tassels on that hebao.

The Huang Jing Mountain overgrown with weeds was their secret playground. They would race each other, catch butterflies together, catch crickets together, and dig for earthworms together. She would say to him: “Ninth brother, slow down a bit” and he would say to her: “A’Heng, hurry up.”

Perhaps it was because their mothers and older brothers forbade them to play together, but both of them were rebellious and wanted to be together even more. They were obviously very good friends, but when their entire family got together, they would pretend not to know each other, but behind other people’s backs, they would smile at each other and secretly make funny faces at each other, pleased that their parents and older brothers did not know their little secret.

When they ate together, because of age, the two would sit next to each other. They did not dare to speak, but underneath the table, he would poke her and she would kick him lightly, both pursing their lips to hide their smiles.

They heard that Uncle Xiang Wan caught a very powerful demon, so they skipped class to see the big demon together. Their heads inched together, and after whispering for while, they came up with countless cunning schemes and actually tricked all the guards to leave. They ran in and unwittingly destroyed the restraints, releasing the fierce and brutal demon. They were so afraid that they ran wildly. A’Heng was wearing a skirt so she was unable to run quickly and was caught by that demon’s claw, and broke her arm. He turned around and saw A’Heng, half of her body was covered in blood, and she was shouting at him: “Ninth Brother, run away! Hurry and run away!”

He was so scared, he really wanted to run, but he was even more afraid that the demon would eat A’Heng, so he ran back to save A’Heng. He jumped towards the demon, waving his hands: “Come at me, come at me! Come and chase after me!”

The demon was infuriated, it abandoned A’Heng and came to pursue him. He could not outrun that demon and was caught by that demon, whose sharp claw pierced his shoulder, while the other sharp claw headed towards his heart. Dragging her broken arm, A’Heng quickly leapt on that demon’s shoulder and forcefully smashed that demon’s eyes, why crying: “Ninth Brother, Ninth Brother. Are you hurt?”

He did not want to be weak like a girl, so he used all his effort to make a funny face at A’Heng. Acting like he wasn’t bothered at all, he breathed in and said: “This demon can be considered quite powerful.”

A’Heng was teased by his funny face that she stopped crying and smiled.

Luckily Uncle Xiang Wan quickly appeared and saved the two of them. Even though their uncle and their brothers all begged leniency for them, their Royal Father was very angry, so he confined them and ordered the physicians to keep their scars, as a reminder of their punishment.

Those days when they studied together, played together, rebelled against their parents together, tricked their older brothers together….

Yi Peng grasped A’Heng’s arm, with an absent-minded expression, as if he didn’t understand how they became like how they are today.

“A’Heng.” Yi Peng called out lightly. Since Third Brother Xuan Yuan Hui had died, he had only been willing to politely call her little sister.

A’Heng’s tears trickled down: “Ninth Brother.” Since Qing Yang had died, this is the first time she has truly viewed him as her older brother.

Yi Peng smiled slightly and said: “If we didn’t grown up, wouldn’t that be great? I really want to go back to how we were when we were little.”

A’Heng’s lingli could no longer fetter his lingli and the pain began to intensify. Yi Peng quietly took the dagger from A’Heng’s waist – that dagger Chang Pu used to commit suicide, and used the last bit of his energy to stab it into his heart: “A’Heng, this time, the demon was too powerful. We all lost.”

“Ninth Brother, Ninth Brother…”

A’Heng called out, frightened and confused. Her entire face was covered in tears, but Yi Peng only made a funny face at her.

That funny expression stiffened on his face and became his eternal goodbye.

“Ninth Brother!” A’Heng hugged Yi Peng, choking with sobs.

On the hillside, butterflies fluttered quickly. There was a young man and a young girl running and leaping in the wind, and their happy laughter rippled with the wind. A’Heng, A’Heng, hurry up, hurry up!

Ninth Brother, Ninth Brother, slow down a bit, slow down a bit!

_______________________________________

[1] Chinese: 氏, used to indicated ‘belonging to the family of’

[2] Chinese: 师傅, teacher

[3] Chinese: 灵气, spirit/power

[4] Chinese: 荷包, a small bag for carrying loose change and other small things.

OTHER DETAILS RELATED TO XY'S MOTHER IN THE NOVELS

When Fourth Prince Chang Yi (Cang Xuan / Zhuan Xu's father) was in danger, Ah Heng begged Shao Hao to send help and even cut off the pinky finger on her left hand and swore a blood oath to try to convince him. Despite Ah Heng's pleas, Shao Hao refused to send help. However, unbeknownst to Ah Heng until much later, he did send a message to Chi You asking him to help Ah Heng. Due to his loyalty to Shennong, Chi You was only able to help Ah Heng with half his strength. In the end, Chang Yi chose to die with his men. Chang Pu tried to kill Ninth Prince Yi Peng to avenge Chang Yi's death, but ended up killing Tong Yu (Yi Peng's mother) instead. Before committing suicide, Chang Pu asked Ah Heng to kill Yi Peng to make sure that Yi Peng wouldn't kill Cang Xuan / Zhuan Xu to avenge the death of Tong Yu. 

Shao Hao kept Ah Heng's pinky finger and turned it into a bone ring that he wore on his left hand for the rest of his life. 

While trying to save Chi You from poison, Ah Heng met the Voodoo King of Jiu Li (an ancestor of the Voodoo King in LYF) and the seventh Flame Emperor of Sheng Nong. The Voodoo King of Jiu Li was a master of poisons who set down his vast knowledge of poisons in the Jiu Li Poison Voodoo Manual, which he gave to Ah Heng. The seventh Flame Emperor of Sheng Nong spent most of his life testing and accumulating medical knowledge, which he set down in the Sheng Nong Herb Manual. Before he died, the seventh Flame Emperor gave the Sheng Nong Herb Manual to Ah Heng. 

This meant that when Ah Heng was living on Cao Yun Peak with Xiao Yao and Cang Xuan / Zhuan Xu, she had both the Jiu Li Poison Voodoo Manual and the Sheng Nong Herb Manual in her possession. Before she "died" on the battlefield, Ah Heng put the Jiu Li Poison Voodoo Manual and the Sheng Nong Herb Manual into a jade container. Then she took Xiao Yao to Jade Mountain and put the jade container around Xiao Yao's neck before leaving to lead the Xuan Yuan army.

On Jade Mountain, the Royal Mother recognized that possessing the Jiu Li Poison Voodoo Manual and the Sheng Nong Herb Manual was dangerous. She made Xiao Yao memorize the contents of both manuals before destroying them. 

Xiao Yao knew how to raise poisonous bugs, and the spells required to plant them, because she had memorized the contents of the the Jiu Li Poison Voodoo Manual. This is one of the reasons that Shao Hao / the Grand Emperor suspects that Wen Xiao Liu (the person who planted a poisonous bug in Cang Xuan / Zhuan Xu) might actually be Xiao Yao. 


HOW THOSE DETAILS ARE SHOWN IN THE DRAMA

In episode 1 of the drama (https://www.youtube.com/watch?v=2grTKaCM14Y&t=1521s&ab_channel=%E8%85%BE%E8%AE%AF%E8%A7%86%E9%A2%91), we see that Xiling Heng has the "Classic of Herbs" and the "Classic of Poisons" in her possession. We see her put them inside a jade container. In this scene we can also see that the pinky finger on Xiling Heng's left hand has been replaced with a false silver finger

Later in episode 1 (https://youtu.be/2grTKaCM14Y?si=vuyfG19JI8iY6jiU&t=1736), Xiling Heng puts the jade container (which is fashioned as a simple necklace) around Xiao Yao's neck before she leaves for battle. 

In episode 13, the King of Haoling asks CX about the physician who had the knowledge and insight to recommend that he use Tanggu water to treat his poisoned arrow wound and use ice crystals to stop the bleeding in the meantime (https://youtu.be/CYUSTBGeil4?si=XOm2swNr7Sqkl9HJ&t=188). Their conversation continues later in the episode (https://youtu.be/CYUSTBGeil4?si=pMNMT_gkJUwa3zt-&t=264). CX mentions that WXL put the poisonous bug in him, and also saved his life twice. The King of Haoling thinks about this. He likely knew that Xiling Heng had the Classic of Herbs and Classic of Poisons in her possession. He realizes that someone who is adept at poisoning, who can manipulate poisonous bugs, who knows about Tanggu water, and who would want to save CX's life twice... that someone could be XY. Even later in this episode (https://youtu.be/3-8jeZtFOmM?si=bY7NndsYrtlVhfvN&t=2102) we can see the ring made from Xiling Heng's finger bone on the King of Haoling's finger.