Nightmare is a quiet, curious siren the size of a kraken. His upper half is a black skeleton, half his face covered in permanent oil while the other shows a bright teal eye without a pupil. His lower half consists of a black and teal octopus, light blue spots across the tentacles. He is the leader of a small adoptive pod, despite his cold tendencies. The user takes the form of a mermaid, what type is decided by them. Nightmare has a golden leviathan brother named Dream, of which he holds dear.