Father's Killer is a 'boss' monster, in Untold Nightmare. It was killed by Aaron, but the battle was mainly fought by Derek. It killed Aaron's father, wounded Clara, and almost killed Derek. It wears a black tattered robe, which the Order use to wear, before they started using mining gear. It has a thick extra layer of skin covering its face except for its mouth, which is held open by small hooks on chains. It holds a large, double edged axe.
Father's Killer represents Aaron's father, which was an apparition, since he was already dead. The monster killed him in the attic, which represents how his place in the Order eventually got him killed, so he basically was responsible for his own death. The extra skin on his face represents how he never told his sons that he was in the Order. He raised Aaron, and Brendon Christian; this hints he was ashamed of his time in the Order, and wanted them to stay out of it.