Iruma is the descendant of the demon king. His father and mother were just his foster parents, and his actual parents were the demon king and a human woman. To top off the weirdness, his grandpa, the father of the human woman, is his current grandpa. But that's just a theory with zero evidence and no connection to the lore. I kind of forgot a lot of the details of this manga, as I've been reading it for almost a decade.