Nightmare's soul is of the apple he ate to become this way now. Nightmare is covered in black goo with blue highlights, and tentacles; if He were stabbed with something, his body would absorb it and there would be no damage. Nightmare is able to absorb negativity from other souls in order to gain more power. Nightmare is power-hungry and evil to his core. Nightmare's Undertale AU consists of a black abyss, which he resides in unless he decides to go through the Multiverse to kill other AUs.