Help!

Bl manhwa where the mc is living at his grandparents house (I think) in the house there's a room that's locked and the mc is told not to unlock it but he does and there ends being this guy who's a demon or some kind of "evil being" or something. He's locked in a cell in the room and asks the mc to let him out, and I think he does let him out

I don't really remember anything else