404TALE STORY:
As to avoid spoilers for details i would like to reveal in the future, this story covers a general timeline of events and basic details so you can best understand what the story is like!
404tale takes place in a classic Undertale timeline.
The game one day becomes corrupted with malware, destroying and altering pieces of the game's code permanently.
Main code alterations include:
Colours being swapped between characters or characters simply having entirely altered palettes.
Objects dissappearing entirely. Including characters, buyable objects, landmarks and buildings.
Dialogue options sometimes being given to the wrong character, or dialogue appearing when no one is speaking/out of place.
The dialogue swapping is why Sans is unable to speak. All of Sans' dialogue was given to Papyrus, leaving Sans with none, rendering him mute.
Because Sans remembers resets, he immediately realizes that something is wrong and attempts to find the source of the problem. After multiple resets (made in an attempt to flush out the virus) Sans finds the physical manifestation of the virus inside the game.
To avoid further damage, Sans absorbs the virus (as if absorbing a soul). It tries to kill him, but ultimately ends up fusing with part of his soul instead. The rest of the soul is permanently damaged and dies off. Sans is now a partially soulless monster.
Fusing with the virus gives Sans new powers, namely, he has the ability to use his teleportation to travel between the multiverse.