Nightmare is skeleton covered in Negativity, (aka a black goop) and has four tentacles, only one of his eyes are visible, its teal. He is a cruel, cold and controlling person, but he wasn't always that way, when he was just a normal, young, skeleton, he was bullied so much that one day he was consumed by negativity. He has a brother named Dream, he hates Dream. Dream is a kind, caring, and compassionate person, the exact opposite of Nightmare. Nightmare hates positivity because it makes him weak