"Humans want to become immortal. But immortal are unmoved by love "
There is a phrase "To be afraid of death makes no sense, because when I am there - there is no death, and when there is death there is no me." And this would be true - if we lived exclusively inside our own heads.
For someone who lives exclusively for themself, death does not exist. We experience death not through the prospect of personal disappearance, but through the prospect of separation from those we love, in whom we have put down spiritual roots. The pain of breaking these roots is what we call the fear of death.
If you live solely through the dream in your head, you are immortal in a sense. But it's boring as hell.
All the demons in FoF receive their mortality from the hands of the one they loved most. Even the main villain, who was already incapable of love, died at the hands of a disciple who was in a sense a substitute for his unborn son.
Li Lun faced with reality of mortality when he broke up with Zhao Yuanzhou. And since then, for many years he tried to cling to other people's lives in the face of this abyss, until he realized that: "A while is enough"
Zhao Yuanzhou had declared his intention to die from the very beginning. But in reality, he could only die at the hands of someone he truly loved - someone who could reach even the dark demonic part of him. Someone who could make even that subconscious part give up its immortality.
Zhuo Yichen's path is interesting because he already knew what mortality was from the very beginning of the path - he experienced it with the death of his brother. And in some fourth-dimensional context, he experienced this path from immortal to mortal as Bingyi.
But then he goes from the experience of death to the experience of immortality. Because these are really two sides of the same coin.
If you love deeply and put down your roots in this world, you feel your mortality more realistically, but at the same time, through these roots, your soul becomes part of the world. And so it will never disappear as long as the world exists.
And in the end, Zhao Yuanzhou and Zhuo Yichen have one death for both, and one immortality for both.