* Golly, this sure is terrible
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Devil!Sans used to be the Sans in the universe known as InfernoTale. Due to him tapping into a deep evil within, his mind and body were changed forever.
Long ago there existed an AU known as InfernoTale, created by God404. He created an AU similar to the universe that would later be known as Undertale. However, it had one major difference. In it, Sans and Papyrus had a deep fiery evil power deep in their souls, called the Inferno. Anyone with the Inferno had the ability to completely tap into its immense inter-dimensional outerversal power, becoming the most powerful thing the multiverse would ever see. However, it would come at a cost: the loss of one's true self. It would turn one completely evil with no remorse. It is nearly impossible to reverse the Inferno's effect on the mind.
In InfernoTale, the human had done a True Pacifist route, befriending the entire underground and restoring hope for all monsters. However, they reset the timeline, this time butchering every single monster. Then, they reset again and attempted to do the same thing. However, this time both Sans and Papyrus remembered what happened. Papyrus, just like before, opened his arms to the human. He knew of their past sins. He knew what all his friends died trying to do. He tried to convince the human to repent. They didn't. They still murdered every single monster. In the end they reset and reset and reset, each time murdering every single monster. Sans never thought to talk to the human, so in each reset, he tried to at least make them suffer a little. The next reset, however, Sans had a plan.
After so many genocide timelines the human, again, approached Papyrus. He spoke to them just like he had so many times. The human attacked. But this time, Papyrus moved out of the way. He dodged many times, but never attacked. Eventually, the human dropped their weapon, and spared Papyrus, being emotionally affected by his words. Sans had watched the whole thing. He was beyond puzzled. Papyrus noticed Sans and cried out to him. "Brother, I saved the human!" Sans could have accepted this as a new beginning, but he didn't. He didn't acknowledge the situation. He instead chose to remember every single time he and his brother were murdered. In that moment, he fully tapped into the Inferno.
His mind and body were changed forever. All he could think was his thirst for vengeance and retribution. As he transformed, Papyrus took the human and ran into Waterfall. Before Sans tried following them, he went on a killing spree, grinding EXP. However, once they entered Waterfall, everyone was evacuated, much like when the human started killing everyone. There was no one to kill, except one: Undyne! She confronted the now Devil!Sans and exposed him for crushing everyone's hopes and dreams. Before Undyne could make her first move, Sans blasted a hole in her chest. Again, akin to when the human did it, Undyne grew in determination and also in power into something far greater.
The one difference, however, was that Undyne was now fighting someone she once considered a friend. In the end, Sans killed Undyne. Papyrus and the human eventually reached the Final Corridor, and Sans caught up with them. The human told Papyrus to keep going while he fought sans. But with Sans' new power, the human stood no chance. What's even worse was that Sans had gained more determination than the human, meaning they couldn't reset or save. Sans knew he would kill the human, so he savored the whole experience and tortured the human excruciatingly before finally severing their head as a trophy. At this point, Papyrus was at the barrier. Sans met him there. He wasn't going to kill Papyrus, he did all of this for him after all, but he was far too dangerous now.
As he inched closer, Papyrus banged against the barrier, crying for help. At that point, a Celestial sans with white glowing eyes and a white robe appeared (The Astral Being). He took Papyrus to a safer place. After Papyrus left InfernoTale, he destroyed all of Infernotale's timelines. Devil!Sans would go on to contaminate the whole multiverse with evil, destroying worlds and tempting others to choose the path he chose.
Other Related Characters
Infernotale - ☀https://undertale-au-fanon.fandom.com/wiki/Infernotale
Devil!Sans wears a raggedy black and red jacket with tears and rips exposing his arms, and a pair of shorts just as raggedy with rips exposing his legs. His skin and horns are red with razor teeth akin to a dragon. His eyes are normally black with a red hue to them, however occasionally in battle his left eye will glow with a fiery orange or green glow. He also possesses a set of black wings which enable him to fly, (although it's likely he would still be capable of flight without them.
Devil!Sans' mind was completely changed by The Inferno, making him a very despicable, and heinous creature. He tries to bring out the evil in people. He even influenced beings such as Murder Sans to choose the path he did. He thinks ANY kind of evil act is okay, as long as it is done for the greater good. He wants this lesson to be known across the multiverse. Although he influences others to choose a certain path, he doesn't care for them. He only really cares for his brother and wishes he would understand why he did what he did. It's still possible to revert Devil!Sans, which his brother is trying to do. There is still a bit of Devil!Sans's old self in him, and some of it shows. He specifically loves Underswap because it reminds him of his home. He still believes his mind has been completely changed for the better and he is most likely in denial that his old self is gone.
Tier: High 1-A | High Outerverse level
Devil!Sans is almost completely invulnerable to all attacks. The only thing that can damage him is God magic which is an energy within beings possessing the Inferno like his brother. Nothing else can damage him.
Devil!Sans exists in all timelines in all universes in the multiverse lurking in the shadows tempting others and pulling the strings without anyone's awareness.
Simply said, Devil!Sans can do nearly anything and create any concept of reality and erase any being lower than him.
Devil!Sans can emit a lethal beam from his eyes, feeding off the life force of a victim's soul.
Devil!Sans has the ability to override anyone's determination in any timeline, gaining the ability to reset.
Omni Timeline Manipulation
Devil!Sans can do literally anything he wants with any timeline. Every timeline altering button is available to him, including Overwrite, Reset, and Erase.
Devil!Sand can alter, obtain, change, and erase someone's code. He can also create code.
Ultimate Inferno Potency
The Inferno was specifically designed to have the potential of incredible good or incredible evil. As a result, it was built to have ultimate power over any being in the multiverse, except The Astral Being, and God404. It contained abilities to take down any Errors or beings with glitched codes, such as Error!Sans, Fatal_Error, Error!Gaster, and both canon and fanon Error404. However, it does not work on God404, viruses or beings with codes containing a virus (Virus!Sans, Virus404, Bluescreen, and Infinity Code), beings with Determination, most Frisks, Undynes, and Charas, or beings with creation abilities (Ink!Sans, Ink!Gaster, Ink!Papyrus, and Creator!Sans), and many other protocols.
- Devil!Sans loves Underswap because it reminds him of his home, although he won't admit it.
- The other characters mentioned in this page from InfernoTale (The Astral Being, and Archangel!Papyrus) will be given their own respective pages (I'm working on it).
- Devil!Sans can't access the Omega Timeline, but he can see everything happening.
- Devil!Sans still cares for his brother.
- Devil!Sans is stronger than Infinity Code, both fanon and canon Error404, and even Multiverse Root Flowey.
- Devil!Sans's creator is God404.
- Devil!Sans has stirred chaos in Reapertale.
- Devil!Sans can hide himself from other Omnipresent beings.
- Devil!Sans revealed himself to Core!Frisk at one time and nearly killed them, but he was stopped by Archangel!Papyrus.
- Devil!Sans has high respect for Nightmare!Sans.
- Devil!Sans is only surpassed by God404.