Underswap is an alternate universe by P0pcornPr1nce, which is under control of the Undertale fandom as of now, just like Underfell. The universe's characters are swapped versions of their Undertale counterpart, for example, Toriel is swapped with Asgore to instead be the one to declare war on humans, Sans is swapped with Papyrus to be a naive person and is overoptimistic to join the Royal Guard, and so on and so forth.
In Underverse, Cross gets sent here by Nightmare!Sans according to their own deal, where Cross must continue to harm the mental will of others for Nightmare's own sakes. However, Cross is immediately taken down by Underfell!Sans and Classic!Sans, thanks to some advice he got back in Underfell's Waterfalls. This could not, unfortunately, stop the oncoming arrival of X-Event!Chara, who is brought back thanks to Underswap!Chara's soul. By overwriting as much as X-Event!Chara wanted to, and Nightmare killing Underswap!Chara, this left the universe without a pacifist timeline. Most of the pacifist timeline's characters have been moved to X-Tale, except Underswap!Sans, who goes to Outertale as his brother and the human are killed when the timeline was erased.
Later on, after X-Tale is corrupted, Cross would head to this AU's pacifist timeline. His first arrival got Cross sturred up, as he would soon realized that the AU's characters remind him of his. He would first noticed with his first glimpse at Underswap!Papyrus; his coding being the same of X-Tale!Papyrus, his dead brother. Cross had enough, and despite being treated with a taco from Underswap!Papyrus, Cross resumes his work on Nightmare, scarring Underswap!Toriel's left eye in the process.
As he was in the process of wrecking the downtown area, Underswap!Sans was busy moving with Underswap!Chara, who would trip over his own cape to drop a canister of paint. As if that wasn't enough, Classic!Sans, Ink, and Underfell!Sans all emerge from the paint as a teleporter device, and Ink is forced to hide the fact that they're actually alternate universes of him by claiming that they're clone about to die soon. Believing this, Underswap!Sans leads them all downtown to find Cross; when they do, he's worried on what Cross's true intentions. He tries to greet him as a friend, but when he tries to attack him, he's attacked back by Sans and Underfell!Sans, and Sans takes back his other half. As all try to leave, Cross!Chara holds on to Underswap!Chara, taking 95% of their soul instead.
Greeting all of them, X-Event!Chara reprimanded Cross's own ideals, and referring towards Ink that he only wanted to see the full X-Event. Sans is immediately in questions regarding X-Event!Chara, but he and Underfell!Sans are thrown out to X-Tale. In the meantime, Nightmare ties up Ink as X-Event!Chara removes all the Underswap characters to X-Tale as well, leaving Underswap!Papyrus, Underswap!Sans, and Underswap!Chara. All 3 of them suffer, thanks in part to X-Event!Chara's overwriting. After he turns Underswap!Papyrus and Underswap!Sans into amalgamate, he urged Ink to hand the other half of XGaster's soul for his own personal game. When he denied them, he used Underswap!Papyrus and Sans against himself before XGaster overwritten them against X-Event!Chara.
Underswap!Sans and X-Event!Chara both go against each other, but due to code similarities with Cross, the latter temporarily stopped their brawl and exposes the danger that was his creator. Underswap!Sans didn't understand what he meant, and both went back to battling, but under the condition that X-Event!Chara kills someone in the process. He first tries to trick Underswap!Sans to kill Underswap!Chara; when Underswap!Papyrus tries to stop him, Nightmare take him towards the breaking point to depression, before reminding X-Event!Chara to kill him. This fails, as X-Event!Chara strikes the ketchup bottle of Classic!Sans. X-Event!Chara is weaken by Sans as he attacks him, and Nightmare is obliterated into goo by Dream!Sans
X-Event!Chara tries to convince Sans that alternate unvierses are completely stupid, and that he'll need to kill "the true enemy" for him. Nightmare had enough with his condition, and he kills Underswap!Chara; the two of them escape the timeline. While Underswap!Sans, Underfell!Sans, Sans, and Ink!Sans leave to Outertale, Underswap!Papyrus refused to join the group, not interested to go along with Ink!Sans after what he thought he done, and the added tease by Nightmare depressed him to the point of accepting his own fate. Dream!Sans apologizes to him as he head back to X-Tale to spy on his brother and X-Event!Chara once more. Once Underswap!Chara finally dies does the pacifist timeline go down with it.
- See Underswap!Sans
- See Underswap!Chara
- super youmna and Jakei both make cameos during the introductory scene of Underswap in Underverse 0.3 Part 1.
- In the long run, the deletion of this AU's pacifist timeline will cause severe consequences to the people originally in place of that timeline, including Underswap!Sans and X-Event!Chara, as he still has much of his human soul from Underswap!Chara
- Description of this video: https://www.youtube.com/watch?v=PhX2TbX7d_s