"In my world of origin there's three kinds of beings who can be summoned, collectively 'daeva', and some dead people become us while some daeva just start existing, and the rest of the dead people go to a different afterlife instead. I'm the dead person kind of demon. Summoning's very common where I'm from, that's why it's so nice. I got, by incredible fluke, summoned to a different world, where summoning was unheard of. The kid who summoned me belonged to a group of people who were preparing to make war on an evil god on another planet from the one they were from.
The evil god was very bad.
Gods of that type can make binding oaths. If they actually say out loud with words that they will do something, they have to.
Mostly he just wanted to keep on being very bad, but the one thing he wanted more was for the other fourteen gods of his type to be dead, and I made a deal with him. There was a lot of collateral damage because in order to kill them I had to destroy the entire planet they lived on, which also had a lot of mortals and about a million mini-gods. I can put the mortals back, they - it's complicated but effectively they count as material objects for me in a way that humans and gods do not, and I can resurrect them, I'm about halfway through doing that on the replacement planet I made, but I can't put back any of the dead gods.
Then I found this place, and it does not void the oath if they get resurrected, so I'm going to stay here till someone can do that."