That’s a dumb demon, demons are in the top 3 trickiest creatures (fey, genie, demons) to make a deal with and the most likely to screw you over. He fell for the impossible mission trap and didn’t remember the constant underlying trap of betrayal.
Damn that's a bit lukewarm. I've been waiting for many chapters and the confrontation ended in 1 chapter with the demon ruler getting tricked and die. At least it was funny how he beg the cultist to send him back. Thank you for your hard work in the scanlation