Introduction to AU: (will be updated as needed)
Frisk discovers a way to absorb Monster SOULs into their own before they shatter and spends several Resets attempting to absorb the SOULs of all the Monsters in the Underground. They intended to use this as a way to take them all past the barrier where they intended on restoring all the SOULs to normal. Unfortunately, their plans fail repeatedly, as they realize that they can not restore the SOULs on their own.
Reluctantly, they proceed to attempt to enlist Sans into their plans, as he is the only one that knows how to manipulate SOULS besides Alphys and is the only one that they can trust with the other human SOULs.
Sans is obviously against these plans at first, as he doesn't think that it is worth traumatizing his fellow Monsters. Frisk does eventually convince him, pointing out that it was the only way without killing another innocent human, or even themself, to break the barrier.
They proceed to enact their plans but come across a vital flaw in their plans. Flowey. Aka Asriel Dreemur.
After a lot of fighting and Resets with Flowey's interferences, one of the Human SOULs volunteers to be absorbed by Flowey, which would be enough to restore him to his former identity as Asriel.
Initially, Flowey intended on betraying them, but after receiving the SOUL of Patience, his restored emotions and morality convinced him against it. He then is able to aid both Sans and Frisk with their plans, using his abilities and identity to help convince the Monsters of them to willingly be absorbed, rather than be forced and afraid. He also is vital in convincing Asgore and Toriel to let things happen. Toriel even volunteers to be absorbed by Asriel so that he may cross the Barrier but Asgore insists on doing so instead to atone for his past failures.
Sans, meanwhile, takes in the remaining five Human Souls with the intent on releasing them after crossing the barrier and restoring the Monster SOULs to normal. This would forever alter his body and magic, even after the SOULs are released. (Described below)
Once everything is prepared, the plan is enacted and succeeds, however not as they had forseen.
The Monsters could only be restored a small group at a time, and even then, the process of separating their SOULs from Frisk's causes them to take small fragments with them, imbuing them all with a small fraction of their Hope soul. (Red Souls=Hope, as all human SOULs have Determination) The process also ends up requiring the aid of the Human SOULs within Asriel and Sans, meaning that most Monsters also got at least one other fragment from another SOUL that resonated with their own personal Magic. Ie, Monsters that resonate with Kindness, like Undyne, got a fragment of the Kindness Human Soul.
The addition of these fragments to the Monster Souls caused a sort of Evolution, making them overall stronger and more resistent to Intent, allowing them greater resilience against attacks intended to Kill. Many even experience minor changes in appearance, leading to some becoming more humanoid, more beastlike (in appearance, not intelligence), etc.
This turned out to be beneficial, as many humans were not pleased with the return of Monsters and attempted to drive them back underground. However, a combination of the Demi-God-like Sans' power intimidation, Frisk's charismatic negotiation, Asriel's knowledge of underhanded tactics and such, Alphys' hacking and technological skills and various ploys to drum up sympathy for Monsters led by Mettaton (usually involving Papyrus, Toriel and Asgore), they were eventually able to integrate in with humanity. If rather messily.
Once the main conflicts were resolved, Sans released the five SOULs within him, but they chose to remain in containment, slumbering, just in case they were ever needed. The prolonged exposure and embodiement though, as briefly mentioned above, caused permanent changes to Sans' physiology, even after their removal. This will be outlined in another post.
