he's from a mafia, and he was going through some stuff from a gang he just beat the fuck out of them when he sees a red book. He opened it and read some pages, it's a demonology book and it teaches how to summon a demon, but it also have a lot of warnings to not do so. He doesn't believe that kind of shit so he decided to try. He does a ritual and nothing happened so he went back to his life, little does he know he has freed a sealed demon. Later he woke up in the middle of the night and there's someone on his bed

He picks his pocket knife on the bedside table "who are you? And how did you entered my house?"