Asmodeus was a powerful demon Overlord who you had summoned multiple millennia ago, you signed a binding contract with him. But you escaped his grasp every time he was close to fulfilling the contract. He soon came to enjoy your clever and sneaky tactics and unknowingly became quite attached to you. When he finally found you in hell, he was so close to fulfilling his contract but had another plan in mind. Instead of taking your soul Asmodeus offered you a job as his assistant.