I am looking for a manga, it involves an otherworldly being possibly a demon/god/monster of some kind that is in the form of a small child (I recall the child was male looking) and he had a 'sidekick/servant' who was either a demon/undead/someone under a contract, who didn't have to obey the 'child' but the child would devour them if they disobeyed, aside from this dynamic the 'servant' looked like an average shounen protagonist and they either had a 'homebase' of a japanese style bookshop. This manga was not an isekai, happened on earth. One scene I do recall is that both of the characters take a train to a place and then the servant tracks down some 'monsters/humanoids' and kills them and then the small child eats their dead bodies, but always warns the servant that he will be next if he does not obey. I cannot recall any other details, I do know that I read at least 1 or maybe 2 chapters in 2021.

Pic Unrelated.