Deciding whether to spare Xanthe and make a deal in Blood of Dawnwalker is a really tough choice. On the one hand, she's an evil ancient vampire who kidnapped and killed a bunch of innocent women, but she was only using Brencis as a tool of convenience. That might be enough for you to ponder what advantage an alliance with her could bring.
Obviously, this guide will contain spoilers for Xanthe's The Cycle of Love quest, but otherwise, here's the outcome if you spare her.
Should you spare Xanthe in Blood of Dawnwalker?
The main impact of sparing Xanthe in her mansion, when you defeat her during The Cycle of Love quest, is that during the final assault on Griefberg Castle:
- You won't have to fight any Blood Guard enemies.
- Xanthe will meet you in the castle and offer some advice.
- You won't have to go through the nightmare sequence you get if you kill Xanthe.
Is this worth it for letting the Vrakhiri live? Not really. If you kill Xanthe, you won't have to fight her in the castle either, so the only real advantage allying with her gives you is not having to fight Blood Guard enemies and not having to complete the short nightmare puzzle sequence with spectre enemies that Xanthe inflicts on you from beyond the grave. If you kill Xanthe and drink her blood, you'll also unlock the Blood Surge ability.
If you spare Xanthe, later on, it's made clear that she intends to keep killing women to keep her daughter Chariklea alive, despite it being quite clear when you meet her during The Cycle of Love quest that she's basically a mortal who's been kept alive against her will, through dark magic and harrowing surgery—all because Xanthe, in her immortality, was unable to let her go.
If you kill Xanthe, you'll then have to decide what to do with Chariklea. Either leave her alone, in which case she presumably starves in the mansion, or stays there alone until whatever dark magic keeping her alive runs out. Or you can put her out of her misery, which is definitely the more humane of the options available.
