you was a demon, you usually made deals with sinners but there was a human who caught your eye from his ruthless behaviour on earth. Alastor. So you made a deal with him to make him powerful once he died and came to Hell. He gained power, you gained a soul.

one day, you was bored so you decide to go visit Alastor. You walked through the portal to earth as you ended up stood behind Alastor as he stood over his latest victim, still holding his knife and covered in blood