...Whats with the spirits of this world allowing deals to be made that results in creating monsters? Why in the world did the spirits agree to that deal that turned Saseas dad into this world threatening monstrosity? If he conquered the world and turned everyone into a deadmell, that cant be good for the spirits.
Far as I can recall, there isn't really anything in this series that makes the spirits actually need humans, they make contracts but with Sasae's conditions it seems like they do it more out of boredom/whim than an actual need. So they might not see too much issue with Sasae taking over.
But also, there's no real reason why becoming immortal requires turning
everyone into deadmells either, far as I remember. If the placating the spirits was an issue, Sasae could just do something like enslave everyone and still have them perform contracts with the spirits or whatever they need, instead of indiscriminately turning everyone into zombies. Just turn the necessary slavemasters into deadmells, keep the rest human.