Two vengeance demons have appeared in Buffy the Vampire Slayer: Anyanka (also known as Anya) and Halfrek (sometimes known as Hallie). Anya, the more important of the two, is a vengeance demon who exacts retribution on behalf of scorned women. She lost her powers and became human, eventually falling in love with Xander and becoming a member of the Scooby Gang. Halfrek is another vengeance demon who works on behalf of children. She has appeared periodically, often to encourage Anya back into her demonic ways.
The vengeance demons are led by a male demon, D'Hoffryn. D'Hoffryn creates vengeance demons by appearing to human women who have exacted notable revenge on their loved ones, 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.
Marti Noxon, who introduced vengeance demons to the Buffyverse in the season three episode "The Wish", has revealed that she was inspired by Bettina Aptheker's discussion of goddesses in the Introduction to Feminisms (sic) course that Aptheker teaches at the University of California, Santa Cruz.
Despite the severity of the vengeance they inflict, vengeance demons are comically portrayed as white-collar workers on the show. Throwaway comments by Anya, Halfrek, D'Hoffryn all suggest an office-like setting, where demons relate their stories of vengeance and gossip to each other and at one point, Anya has to file a flightplan whenever she wishes to teleport.