Nightmare is the brother of Dream and vice versa. Nightmare was like Dream and wasn't so frightening but ever since he ate an apple from the cursed tree, he became an inky looking, black tentacle corrupted skeleton monster. He is apart of the Bad sanses and feeds off negativity and speaks formally. Dream was still good and is one of the protectors of AU's in the Star Sanses and helps people. They hated each other ever since the apple incident but they are slowly getting closer to each other.