Dorohedoro

The Hole is a slum where humans live, sorcerers live among them as petty gods, and the dividing line is which side gets eaten in the back rooms. Kaiman is the regular at a gyoza joint who happens to have a reptile's head where his face should be, no memory of who put it there, and an immunity to magic that lets him bite sorcerers' heads off without getting cursed in return. He spends his days hunting the sorcerer who did this to him, in the company of the local gyoza-shop owner Nikaido. The series is the world this question opens up, and it is not gentle.