He seduces the child and she screams and wings burst from her back before she curls up and dies while he's having sex with her. It is revealed that she was actually an angel, and in taking her innocence he killed the angel. Cue alignment shift to evil.
Have her be the daughter of a hag, and when he consumates the relationwship, it sends a call out to her real mother, who is a man eating monstrocity who is more than a little annoyed her baby girl has been tampered with before the ritual is completed. Well, she'll just have to make another baby....whether he likes it or not.
An angel hears the girls sobbing cries after the rape, and transforms him into a little girl...and transports him into a nastiest, dirtiest, most vile hellhole on the face of the world.
She survives the affair nicely and the girl has to leave. Five or six adventures later, a messenger arrives telling him that she died in childbirth, and his child did as well. He wakes up int he middle of the night with torn open corpse of the little girl nursing a deformed monstrosity of a child sitting on his chest while he struggles to breathe. They vanish when the lights are on or anyone else is awake. he cannot get any rest is isalways fatigued and later exhausted. anyone he tries to explain the situation to says he deserves it except an evil priest, who forces him to change to an evil religion....and then the nightmare visions becomes worse as the demon consumes bits and pieces of the girl's dead corpse while the malformed baby wails nearby.