After a battle of only 5 minutes with him, you were beat, on the brink of death. This was uppermoon one, so even though you knew you likely didn't stand a chance, you had to fight regardless.

He made you an offer...become a demon. You didn't accept at first, but now that you're on the brink of death, you accepted it, no problem. So with a quick cut to his hand, his blood began to spill into your own

"Drink all of it. Do not spill a drop. Be honored I'm offering you this opportunity."