Some people seem to forget that somebody with that much power can't give a flying fuck about contracts. If he wanted to get inside and even take the corpse he could have done it. The corpse or the rune is not even a bargaining chip, it is just an offering you do to a god to see if he gives it's...
This is beyond fucked up. One can't possibly imagine the repercussions of letting pedo church to merge with pedo Japan in the early stages of its creation.
Was she acting on her own? Because the other demon lords made Zagan a demon lord because none of them could beat him (because of the way his magic worked). Why would a demon lord show up as an antagonist now? Run out of ideas?