Sunlight's Seraphim

Vegas Valentine is... a young boy who was forced out of his home the moment he began to develop full tiefling traits. He was raised by his father until he was thirteen, when a group of cultists following the demon god Asmodeus attacked a caravan with them in it. Descendants of the infernal being themselves, Vegas and his father were taken in and tortured with magic. The cultists had made up their mind: the two would be vessels for summoning Asmodeus's form to the Prime Material Plane.
Their plan went awry, however. They were foolish enough to think that a simple blood ritual and a single human soul could summon the god of demons. No, it conjured much worse. Conducted through Vegas's father, an eldritch horrorterror manifested, destroying the temple and killing everyone, short of Vegas. Although he had been consumed by the creature, his father's mind was still there, and he used the last of it to spare his son.
Vegas was also subject to said experiments. His arm had been slowly corrupted by the true demon king's energy, and it was working its way to his heart. It made it there, but not before local clerics and law enforcement arrived. They were able to remove the arm and heal some of the wounds, but the child was missing his entire left torso. In an extreme emergency situation, a determined priest tore a mechanical arm straight from one of their guardian automatons and transplanted it onto the boy. Using extra technology from a nearby allied dwarven city, Vegas was saved. He now must live with a mechanical left arm, and what is essentially a cyborg body.