Vengeance demons are a race of beings that appear in the TV series Buffy the Vampire Slayer. They are immortal women who travel the world exacting grisly vengeance on behalf of human women who have had their hearts broken by men. Their typical method is to appear to the woman in the form of another human female, and encourage them to vent their spleen about the man who has hurt them. The moment the woman says "I wish..." the vengenance demon can cause her wish to come true, whether it requires gruesome dismemberment or even the creation of a new alternate universe.
There are two vengenance demons in Buffy the Vampire Slayer: Anyanka, a.k.a. Anya and Halfrek, a.k.a. Cecily. Anya, the more important of the two, is a vengeance demon who loses her powers and becomes human, eventually falling in love with Xander and becoming a member of the Scooby Gang. Halfrek is another vengenance demon who appears periodically, often to encourage Anya back into her demonic ways.
The vengeance demons are led, perhaps surprisingly, by a male demon, D'Hoffryn. D'Hoffryn creates vengeance demons by appearing to human women who have exacted notable revenge on their errant lovers, and transforming them into demons. In Buffy, we see the creation of Anyanka by this process, and also D'Hoffryn's failed attempt at converting Willow.
