where to get that quest? i have done all quests in act 3, defeating fallen angels now.

You have to talk to vice-governor Vanessa in a house in Alvaron on the first floor. She will ask you for a potion you need the troll bile for.

You can get a quest from Vanessa Kentworth in southwest Alvaron to help her prepare a spell that will supposedly make her beautiful forever. The troll bile is one of the necessary ingredients for the spell.

oh, ok, i declined that quest at start of the act.