Thousands of years ago, a demon lord signed a contract with a human.
Thousands of years later, the devil found the reincarnated contractor through a contract.
At this point, a series of unspeakable things happened.
Therefore, you must think clearly before signing a contract with a creature as proud, venomous, and possessive as a demon.
(If it were a romance version, it would probably, maybe, maybe be called the Devil's Contracted Lover)