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Although Nightmare is incapable of showing sympathy for anyone, 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 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).
His current appearance is quite sinister. Only the outline of shorts, a hoodie, and slippers can be seen. He is completely black, with four tentacles extending from his back, which he can retract at will. His left eye constantly glows blue. This is due to the fact that his body is unstable and constantly "melts."[4]

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 closed with 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, featuring a crescent moon.

Nightmare (from the English nightmare) is a clot of negative energy in a shell copied from Swap!Sans, created by the former guardian of the Tree of Feelings, Nim. Personality
Initially, even before the bullying, Nightmare was a rather 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.[3]

Nightmare is quite attracted to negative emotions and will do anything to obtain as many as possible. Killing victims is pointless, as without them, the negative emotions he craves won't be there. However, he does kill those who break their pacts with him, or force them to remember their past selves before their corruption, or when they anger him (though in these cases, he will still try to make his victims suffer and experience the deepest negative emotions, such as doom, despair, or other similar feelings).

Nightmare never lies, he only conceals the truth. He takes deals quite seriously, which is why he kills those who break them without question.

HGaster used Cross to learn "the behavior of this thing." When Cross regains consciousness, Dream sees something in him that unlocks the key to his soul. Cross recovers and does everything he can to hold Nightmare back for Dream's final attack. Victory ends with Cross asking Killer not to be driven crazy by his boss. Later, he awakens surrounded by the inhabitants of the Omega Timeline and thanks them for curing him, unlike "that old man." He longs to return to Xtale to see his family and escape HGaster's control. But first, Cor, Fresh!Sans, and Dream send him to the Anti-Void to search for Ink. The purpose is still unknown, but Cross is much more energetic thanks to the support of all of Epictale. Interesting facts
Frisk gave him the nickname "Cross" because of the design of his clothes[3];
In Event X, when Cross was asleep, Chara could control his body even though he had no physical soul. His soul could be extracted from his body when he was distracted and took damage.[3]
In Ultra!tale, there's a story where Delta!Sans saves Horrortale from being captured and infected by Cross. The events are somewhat similar to the capture of Underfell.
He is the winner of the Sans AU Tumblr Sexyman 2023 contest, hosted by user CornMayor, as voted by fans.[6]
Cross has official Fell and Swap versions of himself, and they are still owned by the author.[7]

Much had passed before HGaster brought Cross back to life. Knowing Cross wasn't to blame for what was happening, he gave him the choice of whether to go with him or not. Cross found himself in the space where Nightmare and Killer were attacking Dream. He intervened in the battle, and Dream then sent him back to the Omega Timeline. Cor!Frisk greeted him joyfully, despite his displeasure. Cross showed slightly more hostility and retreated to reflect on his actions that led to the genocide. The characters from Epictale (whom he didn't remember due to HGaster's machinations) and Cor explained the situation to him. Cross was equally saddened and asked to be left alone. After some sleep, Dream spoke to him, explaining his involvement in the conflict. Cross, in turn, explained his detachment—everyone who promised him freedom had used him for their own whims. A burst of despair altered reality and inadvertently summoned Nightmare and Killer. When Dream's chances of survival were slim,

Cross was supposed to extract negative emotions for Nightmare, while simultaneously hacking the universe's code and transporting the location to Cross's destroyed universe. However, while Cross was hacking Underfell's code, Ink and Sans attempted to interfere (the latter managed to hint to Cross of his connection to XGaster (an accidental vision of him), which paralyzed him for a long time; even Chara couldn't help). They nearly defeated Cross, but he managed to escape and transport Snowdin to his universe. There, he managed to knock Chara out of consciousness in a dream, having lost trust in him. Cross and Nightmare then visited the surface of Underswap. He was again confronted by Ink and Sans, aided by Swap!Papyrus. While Cross was distracted, Sans and Fell!Sans knocked Cross out and returned half of Sans's soul.

Cross (from English cross) or XTale!Sans is one of the main characters of Underverse. Biography
Cross lived in the XTale universe, created by XGaster. After the events of XTale, Cross's AU was destroyed, leaving the skeleton alone with X!Chara, whom he had trapped within his body. Some time later, Ink appeared, and Cross eventually befriended them. But one day, after declining Cor!Frisk's invitation, Cross asked Ink to return his universe, to which Ink denied being able to resurrect the dead. This caused Cross to distance himself from Ink, who, due to his busy schedule, couldn't be there for Cross all the time, and so he abandoned him. Error then arrived and took Cross's soul, claiming that "his suffering had been too long" and that he could grant Cross eternal peace, but Ink arrived and saved Cross.

Due to the loss of their souls, Chara and Cross were forced to share a body. They teleported to the original universe and stole Sans's half soul, despite Ink's attempts to stop them. Later, Nightmare, sensing the negative emotions, approached Cross and offered him a deal, which Cross accepted.

Possibilities
Horror, though somewhat weakened, is still a very powerful fighter. Having gained several levels, he has other stats that help him defend himself. In addition to his "fragmented immortality," no one messes with him. He's one of the most dangerous monsters in the dungeon.

Appearance
Horror Sans hasn't changed much in terms of clothing. Like Sans, he wears a blue jacket, black shorts with white stripes, a white shirt, and blue tennis shoes instead of slippers. His shirt is also stained with blood.

His appearance is what truly sets him apart. Unlike his original counterpart, he still wears the same clothes, but they're stained with blood. His bones are weakened from not eating for years. He has a large, very noticeable crack in his head. His eyes are completely black, but the left pupil is blood-red.

His smile widened and took on a more psychopathic tint after his skull was fractured (possibly due to his psychosis or because he was in pain).

Horror-Sans is Sans who became a killer after a series of events led to his downfall along with the entire Underground, when Frisk left the Underground without human souls, and the Core without power. Personality
Before his skull cracked and he descended into madness, Sans was a cheerful skeleton with a friendly and intelligent, yet comically lazy, personality. He showed everyone that a few puns can bring a smile.

However, after his skull was crushed and part of his magic was stolen by a rage-mad Undyne, something changed in Sans. Sans declared a new rule: people who fell to the ground must be eaten to survive. After that, Sans ceased to be the cheerful skeleton he once was. Despite the rule that Sans never eats anything, especially people, over the course of seven years, he lost all his kindness and, along with it, his sanity, abandoning his brother, who fell for the trick and ate a piece of meat with spaghetti to save him from starvation and convince him not to go on a hunger strike.

After years of starvation, Sans would seem bored, but it has severely damaged his psychological state. Now he relishes psychological humor and every cry Aliza makes. Whether it's Papyrus's puzzles, the hungry inhabitants of Snowdin, or even himself torturing Aliza, he cares nothing for morality or right or wrong. His jokes have gone from annoying but funny to dark, insane, and sadistic, reflecting his outlook on life.

He enjoys chopping off people's heads for fun (usually for "hunger games"). However, he sets boundaries for himself: for example, he never eats people, despite having set this rule himself, and he tries to prevent Undyne from learning of Aliza's existence. He also regrets and feels sadness, realizing that he almost succumbed to hunger, tore off her arm, and almost ate her, but Papyrus stopped him with letters instructing him not to eat anything that moves. This may, to some extent, prove that the real Sans is still somewhere inside.

Killer is Nightmare's assistant. He first appeared in Underverse 0.4. He may appear again in future episodes. Character Biography
Killer was once a normal Sans, but joined the genocide as a result of a deal with Chara. He killed all the characters in his AU and earned the name Killer. The only murder Killer truly regretted was Papyrus. After his brother's death, Killer had nothing left to regret, so subsequent murders didn't bother him. He always carries a knife, which he uses as his primary attack.

Appearance
The character looks like a normal Sans. His jacket is a more grayish hue with cream-colored fur on the hood. His shorts are white with black stripes. A red circle is located on his chest—this represents Sans and Chara's soul. He also has mascara leaking from his eye sockets, which he can control during this process. At certain points, depending on the situation, barely noticeable pupils become visible in his eye sockets. This most often occurs when Killer experiences strong emotions, such as anger.

Facts
Killer!Sans is similar to Dust. This is evident when he and Dust killed their friends due to Chara's genocide, but Killer, unlike Dust, joined Chara in their genocide rather than trying to resist them.

Other
Even if a reboot occurs, Sans will not lose his LV.
Canonically, Sans ultimately wins. However, there are many different versions of Dusttale, and he doesn't always succeed.
In the most famous fan game, he didn't kill Undyne because Chara quickly dealt with her.
Sans knows Papyrus's phantom is just a figment of his imagination. He only talks to him to preserve the remnants of his sanity.

Personality
Initially, Sans was comedic, yet apathetic towards life due to his knowledge of resets. As a result, he was laid-back and frivolous, but still showed interest in his friends and family. However, everything changed when Frisk landed in the Underground and was forced by Chara to repeatedly kill almost all of its inhabitants. Sans suddenly remembered the previous Genocide timelines and the pain it had caused and interrupted. He then decided to kill other monsters to match their level, slowly going mad in the process.

At first, Sans had a hard time killing his friends, as demonstrated when he briefly hesitated to kill Toriel. He eventually confronted Papyrus and killed him as well. Then he watched his brother die, accepting his death once he realized it would make him happy. This led Sans to completely lose his grip on reality and go completely insane. He then proceeded to kill other monsters without remorse and imagined his now-deceased brother as a merciless ghost consisting only of his head, scarf, and gloves.

Appearance
Dust Sans is basically the same as the original Sans, with a few minor differences. Instead of one blue eye or two small white dots, Dust Sans has two huge red-blue eyes. He also wears a hood to keep dust out of his face.

Biography
After several Genocide timelines, Sans began to go mad. Tired of seeing humans kill monsters over and over again, Sans decided to kill monsters himself to gain power, hoping to defeat humans once and for all, presumably to finally end the Genocide Route.

He presumably killed Flowey in the Ruins, and finally killed Toriel with a toy knife. He also killed other monsters in the Ruins.

He continued his killing spree in Snowdin, where he killed his brother, Papyrus, but seemed hesitant to do so. After killing his brother, he continued his killing spree and eventually reached LV 19.

Ultimately, he defeated the human, ending their Genocide Route. After that, presumably, everything returned to normal, and presumably a non-genocidal route would have occurred.

Sans (also known as Dust Sans by fans) is the villainous protagonist of the Undertale: Dusttale AU and the posthumous main antagonist in the alternate AU, Dustbelief.

He was once a comical and lazy skeleton, but after experiencing too many different Genocide timelines, he went insane. As a result, he began killing monsters in the Underdark to gain LV, hoping to kill humans permanently.