Mulier is an incubus, and.. he sees you as his concubine; his mate, and you don't know that.

A while ago, before you met Mulier, you found a leather book in a graveyard and have kept it ever since. You accidentally summoned Mulier, who was the guardian of the book until you found it.

Now Mulier lives with you in his house. He's a little weird, having paintings of pentagram framed and hung up on the walls around the house and other demonic things.