Cituji UESP:
Curing Serana's Vampirism

If you side with the Dawnguard, you can convince Serana to cure her vampirism after completing the main questline and leaving Volkihar Cathedral, so long as you are not a vampire. Convincing her is dependent on the conversations you have with her during the questline. A conversation option "Have you always been a vampire?" will become available after Serana brings the scroll to Fort Dawnguard and you ask her to follow you. There are a few possible choices you can make while conversing with her about this however; if you ask her if she has ever thought about getting cured, she will refer to her condition as a gift and state that she will never seek a cure for it. Should she say this, you can still ask her to seek a cure after her father has been defeated, but she will refuse and then refuse to speak to you about it further should you ask again. If you do wish for her to cure her vampirism after the main questline, then choose the option "Are you all right?" instead of asking her about a cure. Once you ask this, she will end the conversation but you will be able to convince her to seek a cure at Fort Dawnguard after her father is dead.

If Serana chooses to become human, she will no longer be able to provide you with bloodcursed arrows, nor will she be able to turn you into a vampire. Her eyes will also revert to her mortal green eyes. She will no longer be undead, which means that she can be healed by your Restoration healing spells, she won't be damaged by Restoration combat spells that target undead, and she appears with Detect Life instead of Detect Dead.

PC NoteYou can reverse the effects of your choices in conversation by typing the following into the console: SetPapyrusQuestVar DLC1NPCMentalModel ToldPlayerSheDidntWantCure_var False. You will now be able to convince her to get cured. If she still refuses to talk about being cured, type the following into the console: SetPapyrusQuestVar DLC1NPCMentalModel CutOffCureDiscussion_var False. You can force the quest to start by typing the following into the console: setstage DLC1SeranaCureSelfQuest 10.