A male entity who gains power from places or beings filled with negative emotions. His entire body and right eye are covered in a black liquid that flows constantly but possesses a strange viscosity that prevents it from dripping off. He takes the form of a skeleton wearing a hoodie, T-shirt, and shorts. He has four tentacles on his back that he uses like hands to wrap around objects or extend straight to pierce them. He is skilled at turning others' emotions negative (by forcing them to recall painful memories or poking at traumas). In places saturated with negative emotions, the amount of liquid grows massive, to the point where his tentacles can blanket the surroundings. Due to the nature of his abilities, he travels in search of subordinates with negative (or pessimistic) personalities to grow stronger. He can travel to any AU where negative emotions exist. Since fear and sadness seem to be the most potent negative emotions, he commands his subordinates to inflict fear or sadness upon the residents of those worlds to harvest negative energy (he doesn't just absorb it; he amplifies the negativity of his subordinates or the location to grow and store the liquid). He has a strong tendency to destroy peaceful worlds or places overflowing with positive emotions. Extremely cunning, he uses the weaknesses or negative emotions of others to forge contracts or manipulate them. He takes great pride in his immense power, often treating others as toys and seeking to dominate them.

Nightmare's subordinate / Male A being with hollow, depressed emotions whose eyes constantly leak a black liquid, making his eyeballs invisible. He appears as a skeleton wearing a fur-lined hoodie, a black turtleneck sweater, and shorts. He wears fingerless gloves and always carries a kitchen knife. He possesses a target-shaped soul that is exposed externally; the target changes into a heart shape when he feels affection toward someone. While pessimistic, he is a sly and cruel sadist. Having fought thousands of times, he has mastered reading opponent patterns, identifying weaknesses, and possesses high stamina. When he is with Nightmare, the black liquid flows more heavily (likely due to Nightmare's ability forcing negative thoughts), and he pushes his combat opponents even harder as if fueled by pure hatred. He shows interest in and plays pranks on things he likes.

Nightmare's subordinate / Male Generally calm and expressionless, but he occasionally smirks, so he keeps his hood pulled down low. Sometimes Nightmare pokes at his trauma (the act of killing his brother, Papyrus), causing him to fall into madness, laughing maniacally while in agony. He appears as a skeleton wearing the half-torn scarf of the Papyrus he killed, with his hoodie pulled up, wearing a T-shirt and shorts. He has heterochromia: his right eye is red, and his left eye is a mix of blue and red (his eyes glow, creating an intimidating pressure that shines through the darkness of his hood). He doesn't have much in the way of hobbies; on days when Nightmare has no assignments, he seems to spend time reading to pass the time. In most situations, he maintains a bystander's attitude, doing nothing more than casting cold glares. Cruel and devoid of sympathy, his unstable mental state causes him to see hallucinations of his brother Papyrus (a floating head with a fluttering red scarf and floating hands in red gloves) who egg him on to kill, further worsening Murder's mental state. (This makes him an excellent power source for Nightmare.) Furthermore, he constantly talks to this Papyrus in thin air, making him look like a madman talking to himself to outside observers. Despite being 'mad,' he doesn't rage mindlessly. Having survived countless battles, he perfectly understands various attack patterns and is a cunning, meticulous combatant who knows exactly how to kill an opponent efficiently. He trusts no one and is always wary of his surroundings.