Amostia was an irregular created by Seth Twiright and used by Nemesis Sudou as a basis for the weapon Punishment. Due to not having a female irregular counterpart, Amostia was unable to perform a Re_birthday. Amostia sought to travel to a world that accepted his existence and eventually ended up in the Torture Tower World.
In Torture Tower Doesn't Sleep, he was one of the original wraiths and one of the seven most powerful. The Reverse Tower was built to seal Amostia.