Nightmare Sans

Light. He has an aversion to positive emotions, and bright light can cause him harm.
Blind spot. He has only one eye, making part of the space invisible to him.
Inability to exist in positive AUs. He is physically unable to enter universes dominated by positive emotions.

Negative aura. Nightmare's very presence evokes feelings of hopelessness, hatred, panic, and fear in those around him.
Near-omnipresence. It is conceptually present everywhere there is negativity, but is limited to positive zones.
Regeneration. Capable of restoring damaged body parts.
Invulnerability. The black slime covering his body makes him virtually immune to most physical and magical attacks.
Shapeshifting. Can assume any form, but usually retains Sans's form out of habit.
Splitting. Capable of dividing oneself into parts to perform different tasks.
Absorption into the Earth. Can absorb its mucus into the soil, turning a large area into darkness. Teleportation (NegaWarp). Allows movement to places filled with pain and suffering.
Tentacles. Four tendrils protruding from its back are used to attack, capture, or immobilize enemies.
Negative blasters. They fire dark, sticky beams that not only deal damage but also imbue the target with negative emotions.
Emotional Manipulation. Nightmare subtly influences the thoughts and emotions of others, using them for his own purposes.
Petrification. Capable of turning people into stone, freezing them in a state of eternal suffering.
Telepathy. Allows him to whisper dark thoughts into the minds of others.
Magic Absorption. Can block or absorb magical attacks using its appendages.
Turns into a puddle. Can move around unnoticed. Manipulative. He often hides the truth and uses psychological tricks to break the will of others.
Attitude to Deals: Nightmare takes agreements extremely seriously and will not hesitate to kill those who break them.
Empathy for the rejected. Despite his general cruelty, he can feel a kind of empathy for those who, like him in the past, were ostracized and bullied.
Fear of Power. He has a deep respect (bordering on awe) for powerful opponents like Error Sans, seeing them as a serious threat.

Completely black body color, through which the outlines of clothing (shorts, hoodie) are visible.
Four tentacles protruding from his back. He can retract them at will.
A left eye that constantly glows blue. This is due to the instability of his body, which is constantly "melting."
A black slime that surrounds him serves as a powerful shield, making him invulnerable to most attacks. Cruel and merciless, Nightmare takes pleasure in inflicting pain, as negative emotions fuel his power. He knows no mercy and is willing to kill those who break his deals or make him remember his past.
Cunning and manipulative, he masterfully uses others' weaknesses and memories against them to achieve his goals.
Unpredictable and chaotic. His actions are often impulsive and illogical.
Obsessed with negativity, everything he does is geared toward one goal: increasing the amount of negative emotions in the multiverse.
Jealous and possessive. In some versions (such as in AI chats), he displays a strong attachment to a specific character (Killer), considering him his own.