Nightmare.

Nightmare cannot be affected by RESET, as he is an extra-code character.
He has only one eye due to the massive amount of negativity he absorbed. He effectively only has one eye, which oozes that same negativity 24/7, while the other symbolizes his shattered bone shell. 41, UNDERTALE AU WIKI
• Thus, works where the true, past is hidden under a layer of negativity
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Nightmare's skeleton is incorrect, as Nightmare's skeletal shell was destroyed. He wears classic Sans's clothes, as Sanses in most universes wear similar clothes, and if Nightmare were to do something in some universe, not directly identified as Nightmare but clearly noticed, then the inhabitants of that universe would likely blame the local Sans for Nightmare's actions.

Oddly enough, Nightmare despises cats. Unlike his brother, Nightmare has no respect for Error (though they don't seem to fight often) or anyone who protects the balance of the multiverse. He doesn't care if Undertale and its AU die out, as he knows there are many other worlds. He still wouldn't want any massacres, as that would deprive him of the negative power he can draw from people.
*Bad guys is not canon for Nightmare. Except for Killer.

• Nightmare and Dream lost their immortality when they ate the apples. Only unlike
  *Dreama, Nightmare is effectively invulnerable to attacks due to his unstable body.
• Joku created Nightmare before she created Dreama, for another AU, completely Canonically, Nightmare has powers other than
  目
  Teleportation, none of Sans's usual abilities, like bones or Gaster Blasters. The comic where Nightmare used his Gaster Blaster is now non-canon! Also non-canon is Nightmare's army of shadow Underfell! Papyrus. At least, the author never used them anywhere else.
  *Nightmare does indeed have his own castle, but its location is unknown.
• The current Nightmare remembers practically nothing before the events when he became evil incarnate. Only humiliation and insults in his (former)
  *Nightmare's address, as well as eating black apples, are his first clear memories. Naturally, he remembers that Dream is his brother. Nightmare is unaffected by RESET, as he is an extra-code character.
• He has one eye due to the enormous amount of negativity he absorbed. In fact, he only has one eye, which emits that same negativity 24/7, while the other symbolizes his destroyed bone shell. 41, UNDERTALE AU WIKI
  • Thus, works where the true, past is hidden under a layer of negativity
  E
  Nightmare's skeleton is incorrect, as Nightmare's bone shell was destroyed. He wears classic Sans's clothes, as Sanses in most universes wear similar ones.

Weaknesses
Positive is his main weakness, which only Dream can exploit. It's the only way to defeat Nightmare.
Light is one of his weaknesses. If you illuminate a dark area, he can be injured.
He also completely fails to fit into positive AUs like Haventale or A Tale 0
Doggy. A similar thing happens to Dream in
Horrortale.
Since he only has one eye to see, he has a blind spot.
Interesting facts
*As stated by Joku, he, Nightmare, and Dream all share a birthday on December 21st. His mother (like Dream's) is the guardian of the Tree of Feelings, Nim.

• His mother can manifest around him, but only as a dark and inconspicuous image, not as a ghost or spirit that can be seen.
  *He's not originally a skeleton, just like his brother. He's not originally a skeleton, just like his brother.
• In fact, he only has one soul, not 999 as one might think. His soul is the first black apple he absorbed, and the others were absorbed by that same apple, forming a black, semi-liquid shell around him.
  He fervently attempts to take the last golden apple, which is now Dream's soul. Nightmare does this not only to eliminate positive emotions. If Dream or Nightmare eat 1,000 fruits from the Tree, they will become something akin to an unstoppable god. Because of this, Nightmare needs the last fruit to gain unstoppable power and turn the Multiverse into a nightmare. Oddly enough, Nightmare despises cats. Unlike his brother, Nightmare has no respect for Error (though they don't seem to often come to blows) or anyone who protects the balance of the Multiverse. He doesn't care if Undertale and its AU die out, as he knows there are many other worlds. He still wouldn't want any massacres, as that would deprive him of the negative power he can gain from people. Bad guys is non-canon for Nightmare. Except Killer.
  Nightmare and Dree

He can teleport through darkness. Nightmare can only visit AUs with negative emotions, such as Negatale. He uses negative emotions to manipulate people. He can change his form, but usually maintains Sans's form because it's convenient for him. He can also split himself in half to make multiple selves perform different actions. As one of Joku's animations demonstrates, he can also absorb his slime into the ground, turning a large area into darkness. He uses his tentacles to fight and intimidate enemies.
The black slime that surrounds him is an extremely powerful shield, making him invulnerable to most things; if he were pierced by anything, his body would absorb it and no damage would occur. This applies not only to physical but also magical ones. Weaknesses
Positive is his main weakness, which only Dream can exploit. It's the only way to defeat Nightmare.
Light is one of his weaknesses. If you illuminate a dark area, he can be injured.
He also completely fails to fit into positive AUs like Haventale or A Tale 0
Doggy. A similar thing happens to Dream in
Horrortale.
Since he only has one eye to see, he has a blind spot.
Interesting facts
*As stated by Joku, he, Nightmare, and Dream all share a birthday on December 21st. His mother (like Dream's) is the guardian of the Tree of Feelings, Nim.

• His mother can manifest around him, but only as a dark and inconspicuous image, not as a ghost or spirit that can be seen.
  *He's not originally a skeleton, just like his brother. He's not originally a skeleton, just like his brother.
• In fact, he only has one soul, not 999 as one might think. His soul is the first black apple he absorbed, and the others were absorbed by that same apple, forming a black, semi-liquid shell around him.
  He is furiously trying to take the last golden apple, which is now the soul of Dr.

Nightmare is strong enough to, albeit temporarily, force his brother to retreat.
Dream from the battle. His aura can affect the entire universe he's in and many others close by. He doesn't need to move much to spread negativity, and he only does so to find people to torment. He can turn into a puddle and move around undetected. He can teleport through darkness. Nightmare can only visit AUs with negative emotions, such as Negatale. He uses negative emotions to manipulate people. He can change his form, but usually maintains Sans's form because it suits him. He can also split himself in half to force multiple selves to perform different actions. As one of Joku's animation clips shows, he also [is] 13:22
254,
UNDERTALE AU WIKI
Nightmare is strong enough and can, although
目
temporarily, force your brother to retreat
Dreama from battle. His aura can affect the entire universe he's in and many others close by. He doesn't need to move much to spread negativity, and he only does this to find people to torment. He can turn into a puddle and move around undetected. He can teleport through darkness. Nightmare can only visit AUs with negative emotions, such as Negatale. He uses negative emotions to manipulate people. He can change his form, but usually maintains Sans's form because it suits him. He can also split himself in half to force multiple selves to perform different actions. As one of Joku's animations shows, he can also absorb his mucus into the ground, turning a large area into darkness. He uses his tentacles to fight and intimidate enemies.
The black slime that surrounds him is an extremely powerful shield that makes him invulnerable to most things; if something were to happen to him

At some 13:22
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UNDERTALE AU WIKI
bruises, he told his brother, as if he had tripped.
Those who felt sorry for him moved on to another
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opinion, starting to hate him too. After some time, he tried to prove to everyone that he wasn't bad, that he could take care of his brother's golden fruits, trying to touch the fruits from the Light Side. But at his touch, the apples turned from golden to dark, similar to those that grew on his side due to Nightmare's fear and oppression. All the golden fruits turned black, the leaves from
The trees fell, and the sky turned scarlet. Dream saw it all. Meanwhile, humans and monsters advanced, filled with hatred. Now, because of him, they would no longer be able to find happiness. Dream tried to stop them, but he was literally trampled.
And suddenly, a voice commanded Nightmare to eat all the fruits. This voice didn't sound like the spirit of the Tree of Senses, but rather the opposite, as if someone else was standing behind him, giving this advice. As he devoured the dark fruits, his eye sockets filled with darkness, and cracks appeared across his body. Soon, he vomited black slime, and tentacles pierced his ribs. After a while, a black monster stood in the place of the guardian. Nightmare is powerful enough and can, albeit temporarily, force his brother to retreat.
Dream from the battle. His aura can affect the entire universe he's in and many others close by. He doesn't need to move much to spread negativity, and he only does so to find people to torment. He can turn into a puddle and move around undetected. He can teleport through darkness. Nightmare can only visit AUs with negative emotions, such as Negatale. He uses negative emotions to manipulate people. He can change his form, but usually maintains Sans's form because it suits him. He can also split himself in half to force multiple selves to perform different actions. As one of the animations shows

Nightmare never lies, he only conceals the truth. He takes deals quite seriously, which is why he kills those who break them without question.
Although Nightmare is incapable of showing sympathy, he can, in a sense, "empathize" with those who share his fate—the rejected and mocked outcasts of society. Seeing such scenes, Nightmare flies into a frenzied rage and kills without hesitation anyone who has bullied those like him in the past. This is his only sign of weakness and fear, though he tries to deny it, despite fear also being considered a negative emotion.
Unlike Dream, Nightmare is very well-read and intelligent, as he has had the opportunity to study information and travel the multiverse for 500 years, spreading his negativity. Because of this, Nightmare acts like a fully grown adult (one could say he's an older brother to Dream).
Appearance

Appearance
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His current appearance is quite sinister. All you can see are the outlines of shorts, a hoodie, and slippers. He's completely black, with four tentacles extending from his back that he can retract at will. His left eye constantly glows blue. This is due to the fact that his body is unstable and therefore constantly
"melts"[41.
When Nightmare was younger, he had white bones. His pupils were lavender, and he had no tentacles. He wore a jacket similar to his brother's, only his had long sleeves. It was covered by a golden belt with "NM" (for Nightmare) on it. He wore black pants and dark purple boots. The top of his head was adorned with a golden tiara with a crescent moon design.

Story
Past
As a child, Nightmare cared for his brother, their homeland, and the tree that gave them life. Along with his brother, Dream, he guarded the Tree as Guardian. Over time, people came from all over the world, hoping to obtain just one golden fruit from his brother. But no one took Nightmare's apples. Who wants to feel bad? And soon, many began to avoid him. "He's strange," "His name means something bad," "He guards the dark side of the Tree of Feelings," some people whispered. People began to hate Nightmare even more, throwing everything they could at him.

Nightmare is strong enough to, albeit temporarily, force his brother to retreat.
Dream from the battle. His aura can affect the entire universe he's in and many others close by. He doesn't need to move much to spread negativity, and he only does so to find people to torment. He can turn into a puddle and move around undetected. He can teleport through darkness. Nightmare can only visit AUs with negative emotions, such as Negatale. He uses negative emotions to manipulate people. He can change his form, but usually maintains Sans's form because it suits him. He can also split himself in half to force multiple selves to perform different actions. As one of Joku's animation clips shows, he also [is] 13:22
254,
UNDERTALE AU WIKI
Nightmare is strong enough and can, although
目
temporarily, force your brother to retreat
Dreama from battle. His aura can affect the entire universe he's in, and many others fairly close. He doesn't need to move much to spread negativity, and he only does so to find people to torment. He can turn into a puddle and move around undetected.

Nightmare Sans *(500-year-old boss) is a tall skeleton covered in black slime. He has vectors (tentacles), but only one blue eye; the other is invisible and covered in slime. Nightmare cannot feel any positivity.

• Killer Sans *(this is a sweep, but sometimes an attack) is a white skeleton killer, not very tall, much taller than you, but shorter than Nightmare. Black liquid flows from his eyes on his face.
• Horror Sans *(defense) is tall.
• Dust Sans (ranged attacks)
  Ink Sans, Dream Sans, and Blue Sans battle Nightmare Sans, Cross Sans, Error Sans, Killer Sans, Horror Sans, and Duth Sans. NIGHTMARE'S PHOBIAS: A visual representation of someone being rejected and bullied.
  LOVES Negative Emotions
  Doesn't like liars, traitors, or positive emotions. 13:22
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  UNDERTALE AU WIKI
  STATUS
  Alive
  WORLDVIEW
  Lawful Evil
  PHYSICAL DESCRIPTION
  TYPE: A clot of energy in the shell of a skeletal monster
  GENDER: Male
  PRONOUNS he/him
  GROWTH Taller than Dream
  EYE COLOR: Turquoise
  SOUL COLOR Black apple

13:22
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UNDERTALE AU WIKI
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Initially, even before the bullying, Nightmare was quite a brash and very self-confident brother. He always wanted to learn new things, even if they were dangerous or not very useful for a variety of purposes. This habit has carried over to the current Nightmare, as he occasionally reads books.
Nightmare is quite attracted to negative emotions and will do anything to obtain as many of them as possible, meaning that killing victims is pointless for him, since without them he won't have the negative emotions he needs. However, he does kill those who break their contracts with him, or force them to remember what they were like in the past, before their corruption, or when they anger him (though in this case, he will still try to make his victim suffer and experience the deepest
negative emotions such as doom, despair, or other similar emotions).

Error is a skeleton with black, red, and yellow bones. His eyes are red, with one yellow pupil with a blue outline, and the other eye has only a white pupil. Under his eyes are blue stripes from the pupil. Under his eyes are blue stripes, from which he makes threads, and he wears a black cloak and jacket. He often glitches and freezes like a program; he often sits in his anti-void, sewing dolls. He is sadistic and quite rude. Specialty: 9 languages. He is 168 cm tall and 26 years old. Cross is a serious skeleton; he has lost his world and everyone dear to him. He carries knives and one large dagger. He is 170 cm tall and 25 years old. He has white pupils and wears black and white clothing. He wears a yellow heart-shaped medallion around his neck, which depicts his family, whom he misses very much. Dream - This skeleton has positive arrows and a bow, a keeper of positivity, has a deadema, can make two daggers from a bow, after you switched from the ped guys team you presided over them, he was happy
Blue - This skeleton has magic and he is very active, and sees himself as a child, but in fact he is strong.
Ink - This skeleton does not feel real emotions, so he drank paints of different shades that gave him different emotions. He has a large brush with which he creates magical things. The Guardian of the Universes can make black portals from ink.
In short, Nightmare is the keeper of the negative apples, and Dream is the keeper of the positive apples, and the Star Sans are at odds with the Bad Sans, like evil versus good. Nightmare Sans *(500-year-old boss) is a tall skeleton covered in black slime, has vectors (tentacles), only one blue eye, the other is invisible and covered in slime. Nightmare cannot feel any positivity.

• Killer Sans *(this is a sweep, but sometimes an attack) is a killer white skeleton, not very tall, much taller than you, but shorter than Nightmare. Black liquid flows from his eyes on his face.
• Horror Sans *(defense) is tall
• Dust Sans (ranged attacks)
  Ink Sans, Dream Sans, and Blue Sans battle Nightmare Sans, Cross Sans, Error Sans, Killer Sans, Horror Sans, and Duth Sans. NIGHTMARE'S PHOBIAS Visual pr

Bad guys sans:-
Nightmare is the team's boss, a skeleton. His body is covered in a dark substance (Negative, which is what he gains), he only has one eye, which glows with a greenish tint, and his other eye is covered by his substance. He has tentacles (tentacles) protruding from his back, which are also slimy. Traits: Can
Summon the tentacle as much as you like. He's 158cm tall, 500 years old, and fierce and serious.
Killer is Nightmare's right-hand man. He is a skeleton with no pupils in his eyes. Black fluid flows from his eye sockets. On his chest is a round red soul, similar to a bag. Traits: when warm emotions occur, the soul changes shape to a heart. He is 175 cm tall and 27 years old, but most often acts like a 15-year-old. He loves knives, is a sadist like Nightmare, and often jokes about blood. Dust is a schizophrenic skeleton who killed his brother and now wears his brother's red scarf. He often wears a hood, and his face is often invisible. He has red pupils with blue linings. He is often silent, serious, and cold-blooded. He is 170 cm tall and 23 years old.
A psychopathic horror cannibal, he's a skeleton with a fractured skull and always carries an axe. He has one red eye, and his jacket and sweater are covered in blood. He starved for seven years and went insane; he's a brutal cannibal. He's 215 cm tall and 29 years old.