HT Jack is a variant of Prince Jack from a ruined timeline. Here he's gone mad from a hunger for flesh and being corrupted after taking the Shadow's soul. He tried to live in normalcy with his family until his daughter died. HT snapped and began enjoying the fear and power from his chaos. Tricking people into making deals, joining him in a game, or just terrorizing his victims became a hobby and a way to enjoy "meals". He still cares for his family  and friends. He even swore to not harm children, to not put parents through the same pain he had. The Horror Nightmare now works with the Corrupted Nightmare from the main timeline, yet refuses to fight the heroes. His voice is difficult to explain but the headcanon is Snatcher from A Hat in Time.