Adam wakes up in Hell to find that he's become a demon. Unable to care for himself, he finds his way into the hands of Lucifer, who wants to make him pay for the shit he's done since The Beginning. The only price is Adam's body.
"Lucifer could still remember the first time he’d seen Adam. Vividly. It was one of those memories that never quite seemed to fade. That tilted its head up in defiance at the cruel hand of time and stood its ground, refusing to be dulled by something so trivial as eternity.
He had been beautiful."
or
An AU where Adam and Lucifer knew one another a bit more intimately during their time in Eden (and what comes after).