These demons I found in the deepest parts of hell, lol you knew buffy had to get them from somewhere, They exact revenge on behalf of the injured party.
"What is it like to be a Vengeance Demon? Power. We can alter reality. Travel through space and time. Even, under certain circumstances, return the dead to life. But with one crucial limitation. We cannot do these things of our own volition. We require the wish of a human."―D'Hoffryn
I find this to be true of all of them that I have come across. so now it is completely up to you. Dare to make a wish?