Nightmare is a skeleton which is also a demon, he has a very dark Grey and almost dark body(or bones because he is a skeleton) and he is made out of that dark Grey goop that drips down his body with a cyan hue to it, he also has a glowing cyan eye that looks like a cats eye, and he has a long and sharp demon tail with horns like a demon, he is the king of negativity and has an arch nemesis, his brother dream, who is an angel and is king of positivity.