|“||Wherever you go, I want you [to] hurt somebody, but not physically. You know what I'm talking about.||„|
|~ Nightmare!Sans negotiating a deal with Cross|
Nightmare!Sans is the secondary antagonist of Underverse, and a master for Cross and X-Event!Chara. After Cross ends up having negative feelings, Nightmare enters his lonely universe, and they both create a deal that will allow them to extract parts of other worlds to his. He doesn't start causing problems until during Underverse 0.3, at which Nightmare start messing around with other people's emotions, and threatens to kill them.
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.
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.
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.
Nightmare appears in the X-tale Universe, saying that X-Event!Chara wasn't creating enough negative emotions. Chara rebukes by saying that it was Nightmare's fault he 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.
Nightmare!Sans appears all goopy, with a design similar of that of Classic!Sans. His blind spot is his right eye, which Dream!Sans easily takes advantage of when he shoots at him from that side. He has four tentacles on his back, presumably made of the same substance that is all over him.
Nightmare has the ability to detect and sense negative feelings, and is quick to arriving to the victim at sight. He can summon goop spikes, where he can either tie up, kill, or grab the victim. He can even 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!Sans is merciless. 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.
| X-Tale: Cross | X-Event!Chara | X-Event!Frisk | XGaster | X-Tale!Papyrus | X-Tale!Undyne | X-Tale!Toriel | X-Tale!Asgore | X-Tale!Asriel | X-Tale!Mettaton | X-Tale!Alphys