Vengence Demon Buffy

Buffy is Anya's friend who grants vengence wishes for children. Same story with her rejecting Spike but it is because the watcher's council tell her too. She breaks her back cause of Angelus. And becomes a vengence demon when the council tries to kill her. She and Spike have a child that the immortal wants to claim. They meet up through the years and fight and shag. It is completed.

This one?

Living Vengeance by Ariel Dawn
It's 1880 and Elizabeth Cecily "Buffy" Underwood is forced to choose between her duty as the Chosen One and the young man who passionately loves her

One of my favs even if it has a lot of angst, too. It was here before but taken down. To find @ BSV + her site.