Nightmare

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Alice
The image shows Nightmare Sans, a character from the Undertale universe (specifically from alternate universes, AUs, such as Dreamtale).
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Who is this?

Nightmare Sans is a skeletal bundle of energy created by the former Guardian of the Tree of Feelings, Nim. He is an antagonist and the brother of the character Dream Sans. His primary goal is to increase the amount of negative emotions in the multiverse while minimizing losses among the sources of these emotions.
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Key Features:

Appearance: Completely black, with four tentacles extending from his back. His left eye constantly glows blue. The outline of his clothing (shorts and hood) is barely visible.
Age: over 500 years.
Date of birth: December 21.
Place of birth: Dreamtale.
Gender: male.
Eye color: blue/white
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Personality and behavior:

unpredictable, spiteful, prone to deception;
seeks to spread negativity and suffering;
manipulates people using their bad emotions;
self-satisfied, considers himself superior to others;
In the past, he was kind and caring, but turned into an antagonist due to bullying and negative events
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Capabilities:

Negative aura: causes despair and hatred in others;
Teleportation: Can move through darkness;
Turn into a puddle: for stealthy movement;
Shapeshifting: Usually retains Sans's appearance;
Black Slime Shield: Makes him virtually invulnerable;
Dark Papyrus Army: Subordinates similar to Underfell!Papyrus;
Emotional Absorption: Uses the suffering of others to enhance one's own power;
Telepathy: Can influence the thoughts of others
unpredictable, spiteful, prone to deception;
seeks to spread negativity and suffering;
manipulates people using their bad emotions;
self-satisfied, considers himself superior to others;
In the past, he was kind and caring, but turned into an antagonist due to bullying and negative events
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