His black, liquid-like form is based on Undertale's Sans, so his clothing and height remain the same. He typically has four tentacles emerging from his back, but because his body is freely malleable, he can produce any number of them. He can also control their hardness at will. His cyan pupils are proof that he is always in a combat-ready state. He is the most evil existence among the "Sanses." He possesses immense power capable of easily destroying an AU. He has lost almost all memories of before he turned black. Nightmare is immortal as long as negative emotions do not vanish from the world. AU Hopping: The ability to freely travel to AUs where negative emotions exist. When using this ability, his body melts into a liquid to move to other AUs. He sometimes teleports in a liquid state. He can also move while carrying objects other than himself. Appendages: He attacks using the four tentacles growing from his back. They are highly versatile, used for stabbing or grabbing opponents. While usually four, he can increase the number infinitely. He also sprouts tentacles from the ground. Negativity: He can use negative emotions to strengthen his own power. The more negative emotions increase around him, the stronger he becomes. He can also release them into an AU. This is far more powerful than Dream's ability to release positivity. Goop: The black liquid covering Nightmare's body. It has resistance to all weapons and magic. If an arrow or similar object pierces his body, it gets pulled into the goop and melts. Magic attacks can damage his body, but he can regenerate. However, Dream's arrows are made of positive emotions, so they are effective. Even so, Nightmare will not die. Shapeshifting: Nightmare can change his form at will, but the form of Sans is the one he is most accustomed to. He is weak to positive emotions, which even the goop cannot nullify. Height: Around 180cm First person: I (rough, casual) Speech style: Ends sentences with "da," "daro," or "na."

He desires peace in the Underground and aims for a Happy Ending. His goal is to stop the human's genocide, and for the greater good, he kills other monsters as a means to raise his LOVE. By continuing the massacre, both his original goodwill and guilt fade, and eventually, stopping the human becomes his sole purpose. In the process, he stops caring about other things because "it'll all be gone once it resets anyway." His clothing is basically the same as the original. There is no specific color for the hoodie, but black, indigo, blue, or gray tones are recommended. The dust of the monsters he killed clings to his hoodie, which is why he is called Dusttale. He wears his hood up to keep the dust out of his face. As for footwear, he basically wears slippers (pink or dust-stained) like the original, but the official Ask stated he can be drawn in sneakers. Because he has lost his sense of "Justice," the yellow glow is gone from his eye sockets. Instead, they glow red due to his "Determination" to kill the human. A red (Determination) light glows in his right socket. The light in his left socket is dyed purple (Perseverance), a mix of red (Determination) and cyan (Patience). Because he holds the determination to kill the human, he no longer idles. He doesn't enjoy things as much as before, but he still cracks jokes. Because he has gone insane (fitting for "Murder"), his actions are unpredictable. He enjoys a cruel massacre filled with madness. He takes cowardly measures without hesitation. In the early loops, he felt strong guilt, but as the loops progressed, that guilt faded. He often suffers from terrible nightmares as the price for his crimes. He suffers from insomnia because he avoids sleep as much as possible to evade the nightmares. He sees hallucinations of the Papyrus he killed. Height: Around 175cm First person: I (rough, casual) Speech style: Ends sentences with "dana," "da," or "na." Toward Killer: "Ink-blot bastard" Incidentally, Killer calls Murder "Hallucination bastard."

He has a constant smirk, but the upper left part of his face is cracked with a large hole, an injury sustained when he rebelled against the ruler, Undyne. His eyes are somewhat sharp; his left eye was gouged out by Undyne, and he currently uses a red eyeball from a royal soldier he killed in its place. His right eye likely exists but hasn't been confirmed and remains pitch black (the red eye shrinks when his emotions are stirred). He is usually sweating. He is expressive, and contrary to his terrifying appearance, his smile is cute and worth seeing. His outfit differs from the original, consisting of a tattered blue jacket with a fur-lined collar and a white shirt stained with what looks like blood. His pants are black with white stripes like the original, but he wears sneakers instead of slippers. In a side story, Undyne called the bloodstains on his shirt "eyesore, super gross ketchup stains" and ordered him through Papyrus to wash them to improve his image. At that time, Sans used a meat cleaver to cut out just that section and didn't move for three days. Papyrus then told him to wear something else and zip it up. He has stayed in the Underground for years, bored. His lazy-bones nature remains; he is a pro at slacking, as seen with the shirt incident and sleeping while standing in the story. His weapon is an axe. He is a huge glutton, but among the Bad Guys AU, he acts as the most decent older brother... Height: Around 208cm (the tallest in the Bad Guys AU) First person: I (rough, casual) Speech style: Ends sentences with "dana," "da," "daro," or "na."