Goodbye My Princess: Director's Cut Episode 54

Xiao Feng turns eighteen. She goes to Li Chengyin and asks him if he is willing to die with her or if he'll kill her in the future if she poses a threat to his throne. Li Chengyin says that he will not kill her and that it kills him that she doesn't have him in her heart. Xiao Feng tells him that she actually cannot help but have him in her heart and kisses him. Li Chengyin is overjoyed but unbeknownst to him, there was a sedative on her lips and he passes out. Xiao Feng says her farewell to Gu Xiaowu for the last time and escapes with A'Du. She gets a final chance to bid Yong Niang farewell as well. Mi Luo arranged for them to leave town through a Goryeo merchant caravan. Li Chengyin sends soldiers to get Xiao Feng by any means necessary. They meet at the Yumen gate. A'Du manages to get through but Xiao Feng doesn't. She climbs the tower and jumps. Li Chengyin remembers everything. (Source: Autumn carrot at MyDramaList)