RainbowTale is a hacked AU, where an unknown outside force injected the AU with a hack, known as Rainbow.exe . The hack quickly spread through a Classic Undertale, resulting in a separate AU being formed automatically.
AU By: Fandom---Fiction / Conf
Rainbow.exe has three protocols: Infect, Spread, and Purge. Then repeat.
What this meant for the denizens of RainbowTale, well... The first victim was Flowey, an 'easy' target to the hack due to his isolation from the rest of the monsters. Flowey fought back, obviously, however with no soul of his own he was easily overwritten by the Hack. From there, the hack spread throughout the Underground, taking over Toriel, and then Frisk when they fell.
The last to fall was Sans. The most powerful, the Hack found, despite his seemingly low stats. However, he was unable to force himself to strike down his former brother, and as such he fell to the Hack as well...
From there, the Purge phase began. The Hack automatically pitted all of its hosts against one another, seeking to whittle its own population down the the sole strongest host. The final battle was between Frisk and Sans, where both were nearly of equal level after the slaughter, but Sans had superior speed, as well as teleportation and telekinesis. This lead to victory by Rainbow!Sans.
Rainbow.exe gives its final host the ability to travel between AUs and Timelines, so that the process may begin anew, spreading through the AUverse tearing a swath of blood, dust, and color. The original Rainbow!Sans has been through seven unknown AUs, and has yet to be bested. But for how long will this continue?
That is up to you!
Thats right, this is an OPEN AU! You want Rainbow to hit your AU? Go for it! You want to Rainbow one of the popular AUs like Underfell or Underswap? Go for it! Though, be aware, Rainbow.exe automatically creates a separate AU when it begins spreading through an existing powerful AU, so it is not technically capable of destroying an AU for good! I would also ask nobody kill of Rainbow.exe , as that puts an end to other people's ability to make their own stories around it!
List any AUs you Rainbow below with links! I would love to read them!