Nightmare Sans

Capabilities
Power and aura. His aura affects the universes he enters. He can even temporarily force his brother, Dream, to retreat.
Army. Controls an army of dark Papyrus, similar to Underfell!Papyrus.
Transformation: Can turn into a puddle of black goo to move undetected.
Teleportation. Able to move through darkness.
Emotional manipulation. Uses negative emotions to manipulate people.
Shapeshifting: While he can change form, he most often retains Sans's form for convenience.
Territory control. Can absorb its slime into the ground, plunging a large area into darkness.

Nightmare Sans (from the English word "nightmare") is a character from the Undertale fan universes (AUs), a bundle of negative energy in a shell similar to Swap!Sans. He was created by Nim, the former guardian of the Tree of Feelings.
Nightmare Sans Undertale

Appearance
In his current form, Nightmare is completely black, with four tentacles that can retract from his back. His left eye constantly glows blue. The outlines of his clothing (shorts, hoodie, and slippers) are barely visible.
In his youth, he had white bones, lavender pupils, and no tentacles. He wore a long-sleeved jacket, a gold belt with the monogram "NM," black pants, dark purple boots, and a gold tiara with a crescent moon.

Character and motives
Nightmare's main goal is to increase the amount of negative emotions in the multiverse while minimizing losses among their sources. He often deceives, behaves unpredictably, and is vicious.
Nightmare isn't prone to lying, but he often conceals the truth. He takes deals seriously and kills those who break them. However, he can "sympathize" with those who have suffered rejection and abuse, like himself, and in such cases, he becomes enraged.

Story
As a child, Nightmare cared for his brother, Dream, their homeland, and the Tree that gave them life. Together, they guarded the Tree as Guardians. Over time, people from all over the world began to come for the golden fruits Dream provided, but no one took Nightmare's apples. Because of this, he was shunned and even bullied.
The turning point came when Nightmare, tormented by despair and envy, ate a tainted (dark) apple. His own negative emotions corrupted the fruit, turning it into a black apple. In a moment of pain and hopelessness, Nightmare consumed the remaining dark apples to gain power, becoming what he is today—a being of fear, hatred, and negativity. He then destroyed the Tree.