Nightmare Sans

Nightmare is over 500 years old, and during this time he has become almost a godlike force.

It is not subject to Reset because it exists outside the code.

His true skeletal form is destroyed forever - no amount of "returning to the past" will bring back his former self.

He wears Sans's clothes deliberately to mislead - AU residents often blame the local Sans for his actions.

There's a theory that he's a reference to Slenderman from creepypasta.

Nightmare is confident, cynical, and calculating. He doesn't lie, but he hides the truth. He considers himself superior to others, especially those who protect the balance (like Error Sansa).
However, deep down, he still remembers his past. If he sees someone being bullied, he flies into a rage and kills the offenders without hesitation. This is the only place where his hidden weakness manifests itself—the fear of becoming a victim again.

Positive emotions are his greatest weakness. Only pure kindness and light (like that of Dream Sans) can harm him.

Light—physical and metaphorical. Bright light weakens it.

Cats—oddly enough, he despises them. This could be a reference to his dislike of anything cute or harmless.

Nightmare is an incredibly powerful opponent:

Absorbing negativity: He draws strength from the suffering of others. The greater the fear, the stronger he is.

Teleportation through darkness: can appear and disappear at any time.

Damage Immunity: His body absorbs attacks, whether physical or magical.

Splitting into copies: Can create multiple versions of itself to act simultaneously.
Manipulation of emotions: instills fear, despair, paralyzes the will.

AU Travel: Can only visit worlds with a predominance of negative emotions, such as Negatale.

Nightmare was once an ordinary guardian of the Tree of Feelings, Dream Sans's brother. He was kind and caring, but because of his connection to the Tree's dark side, he was shunned. People feared him, mocked him, and called him strange. Over time, these humiliations broke him.
To survive, he ate the Tree's black apples, which were filled with negativity. This energy changed him forever: he became a creature that fed on the fear, pain, and despair of others.

Nightmare looks scary:

It is completely black, like a shadow, with blurred outlines of the body.

He is wearing a hoodie, shorts, and slippers, reminiscent of classic Sans's outfit, but in black.

Four tentacles protrude from its back, which it can use to attack or frighten enemies.
His left eye glows bright blue, while his right eye is blank or dark, symbolizing the loss of humanity.

Nightmare's body is unstable, appearing to "melt" or turn into a dark goo, allowing him to turn into a puddle and move around undetected.