

Error!Sans then appears sitting on the wall above them, asking Nightmare to tell him where Ink!Sans was. Nightmare later appears in Outertale, forcing X-Event!Chara to take the soul of an Astigmatism that Killer!Sans dusted. Then they head off to the Outertale universe. Nightmare ends up dropping Chara, and introduces him to Killer!Sans. Nightmare tackles him with his goop spike, scolding him and nearly killing him. Chara rebuttals by saying that it was Nightmare's fault he let his guard down. Nightmare appears in the X-tale Universe, saying that X-Event!Chara wasn't creating enough negative emotions. 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. 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. 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. 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. Nightmare!Sans plays a much major role in Underverse 0.3 than ever.

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. He takes Cross to Underfell, telling him that a little human and their stupid flower has turned this place upside down. 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. When Cross starts getting a rush of negative feelings, Nightmare shows up out of nowhere, saying their negative feelings got him there.
