🌙 ˖ ࣪ ‹ ( Nɪɢʜᴛᴍᴀʀᴇ! Sᴀɴs ) 𖥔 ࣪

Nightmare!Sans appears with a design similar of that of Classic!Sans, however his body is made out of a black, sticky, tar-like substance. The similarities end here, as his right eye is completely covered in the "goop", which Dream!Sans easily takes advantage of when he shoots at him from that side. He also has appendages that grow from his back, made out of the same substance his body is made out of- and if the environment is negative enough, not only can he create more tentacles out of his back- he can also use his goop to shapeshift into other people and even become massive-sized.

Nightmare!Sans is, in general- sadistic. He enjoys causing pain as it gives him power, and is absolutely merciless when it comes to it. Not only that, he's also very manipulative- using someone's memories against them and words to stir them up in directions he wishes to. While he tells Cross not to kill anyone, he's given X-Event!Chara a chance to kill somebody, and when that fails, he kills Underswap!Chara himself. He even tries multiple times for Underswap!Papyrus to die. Nightmare!Sans also hates broken deals, and would destroy a victim's life for that.

Nightmare has the ability to sense negative feelings, and has the ability to hop through alternate universes. He is made out of black goop, which commonly manifests in the form of appendages he can use to tie up, kill, or grab the victim. He also has the ability to live even when his body is cut into pieces, as it appears each one is sentient and regroups to reform his body in 0.3. As shown in Underverse 0.6, he can also shapeshift into familiar forms in the memory of someone, It also appears that his powers overall grow depending on the negativity in the environment, as in 0.3 he only was capable of doing so much due to the positivity but in 0.6 he can even shapeshift into a much larger form. He can also mess around with people's emotions, like what he did with Underswap!Papyrus. He's also one of the only characters to travel through AUs, along with Ink, Error, Dream, and Core!Frisk. Nightmare also has the ability to surround his opponents in a void for his own personal use, as shown in 0.6. His Aura brings out despair and Hatred out of those that surrounds him.

0.1 When Cross starts getting a rush of negative feelings, Nightmare shows up out of nowhere, saying their negative feelings got him there. Cross!Chara tells him to negotiate a deal with him, in which Nightmare tells him to mentally hurt someone if he wants to steal parts from other universes. He takes Cross to Underfell, telling him that a little human and their stupid flower has turned this place upside down.

0.2 Nightmare!Sans doesn't have a bigger role in Underverse 0.2; he only watches the battlefield from a cliff, and then leaves after Cross!Chara helps Cross escape the attacks from Underfell!Sans. 0.3 Nightmare!Sans plays a much major role in Underverse 0.3 than ever. He first scolds Cross over his actions, and after calming down, he swore to him that he'll make him relive the death of his universe again if they break the deal, before taking him to Underswap. After Cross is taken down and X-Event!Chara is revived, Ink!Sans tries to stop X-Event!Chara from doing his intentions, but he's tied up by Nightmare and isn't able to move freely until he gets annoyed over X-Event!Chara's constant killing of the Underswap cast; later, he allows X-Event!Chara to kill someone.

Throughout part 2, he would threaten to kill Underswap!Papyrus, tells X-Event!Chara to kill himself to kill Underswap!Chara, and once Underswap!Papyrus tries to intervene, he tells X-Event!Chara to kill Underswap!Papyrus instead, which is averted once Sans intervenes. He's then destroyed by his brother, Dream!Sans with his arrows; despite this, he still ends up killing Underswap!Chara when he leaves with X-Event!Chara, knowing that he lost his chance to kill someone. 0.4 Nightmare appears in the X-tale Universe, saying that X-Event!Chara wasn't creating enough negative emotions. Chara rebuttals by saying that it was Nightmare's fault he let his guard down. Nightmare tackles him with his goop spike, scolding him and nearly killing him. Nightmare ends up dropping Chara, and introduces him to Killer!Sans. Then they head off to the Outertale universe.

Nightmare later appears in Outertale, forcing X-Event!Chara to take the soul of an Astigmatism that Killer!Sans dusted. Error!Sans then appears sitting on the wall above them, asking Nightmare to tell him where Ink!Sans was. He is later seen observing Ink!Sans and Error!Sans fighting. During the credits of the episode, he is shown fighting Dream!Sans and Cross!Sans before they both disappear, leaving him alone with Killer!Sans. 0.6 Nightmare plays a key role in 0.6. During the episode, Nightmare opts to mess around with Cross's emotions while Nightmare's assistant Killer fights Dream. After XGaster takes control of Cross and wins against Killer, Nightmare steps into battle. Dream encourages Cross to get up and keep going, but Nightmare taunts them about that “shameful pride” and how they are stuck in his domain. He grabs Cross, and taunts him by transforming into versions of Cross’s old friends. Eventually, Dream gets a shot in with his bow, and Nightmare and Killer are shown leaving after Nightmare claims that he will “always be there” to Cross.

Nightmare and Dream were born from the same tree. Nightmare wants to be like his brother, so he tried to make friends with villagers, but with no avail. Dream has made friends while Nightmare was bullied. beaten and bullied by them for being the guardian of the negative side of the tree. He is seen trying to take care of his brother's apples. Later on, as the bullying gets more intense, he thinks his apples are not good enough and are looked down upon. His brother reassured him that his fruit is just as good as him.

People traveled from far and wide to get just one golden fruit from his brother. Nightmare envied his brother because no one would take fruit from his side of the tree. One day, he took an apple from his brother's side, to show he could take care of it. But a being who is filled with negative feelings will turn a positive apple into a negative one. When trying to take care of one of Dream's apples it turns black like Nightmare's fruit. Because of this, an angry mom storms around him and started attacking the poor guardian. As Dream came back to protect his brothers, Nightmare was tempted by a vengeful spirit and started to devouring the negative fruits. The overwhelming power damage his body and purged his spirit, allowing the Evil Being to take full control of the vessel.

As stated by Joku, both his and Dream's birthday is December 21. December 21 is the first official day of Winter and the last day of the zodiac Sagittarius. This makes both Dream and Nightmare Sagittarius. Joku stated that Nightmare and Corrupted Nightmare aren't the same person, even though they shared the same body.