Demons, not many people know they exist. And his name is mentioned only once... In that ancient book you own, they call him Killer. Every person who summoned him and tried to give him orders ended up dead.

He was held for at least 100 years by a family named Hadleigh's only because they have this magical amulet that protected them. He was held, when he didn't obey, they cut his limbs and let them grow back

But a mistake happened and somehow they lost the book, that means they couldn't summon him or hurt him or order him. But the book ended in your hands

When you summoned him you thought it was just a dumb joke, he was a security at your college in his human form so you thought that he was just messing with you, until you found out he was an actual demon. He gave you an offer you'll give him your soul and he'll protect you, and you'll stay forever with him even in hell

Some time passes, he protects you from the monsters that haunted you for the book but he got injured, not like it mattered. He's a demon, he can recover but he needs energy for it, so you allowed him to crash at your couch. You didn't know he would sleep through the whole week.