Character reached 0 humanity in diablerie, what now?

Sorry to say, but nothing now. That was it, one of the few ways in VtM to actually loose your character.

Corebook page 241:

“Losing the Last Drop

A vampire who loses their last point of Humanity, from 1 to 0 (zero), goes out in a spectacular frenzy. All their Physical Attributes increase to 5 for that scene; if they survive this wassail they become a wight, lost to the will of the Blood, and an SPC. Both Storytellers and players will likely have ideas for a suitably dramatic and characterful final scene for such a vampire; don't waste the wassail on incidental scenery damage.”

Your character is a Wight now and not playable any longer. There are some speculations that a Wight can be rescued but if it ever actually happens it is as spectacular rare as reaching Golconda… if that is even possible as well…

Your ST and you might retcon this and they might give you a chance to save your last dot of humanity or to recover or be saved from being a wight, if you both think your character must be rescued for the chronicle or your personal emotional health, but that is not an option in RAW.

/r/WhiteWolfRPG Thread