Fell Sans in FGO/Fate Grand Order: Rubrum Mortuus Fatus

Fell AUs were notorious throughout the Undertale Multiverses to be some of the cruelest universes ones out there. The sheer disorder that lay in them was so intense that it burned with a never-ending fire, the residents of them being the same.

Hell, it didn't help most were evil, no redeemable qualities in them. Sans was often cruel, no compassion or mercy in him.

One day, one version was pushed to the brink and caused a universal collapse. Because of this, the void swallowed everything whole, but he wasn't dead. He was still "existing", even if others didn't know.
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Fate AUs were a fickle thing. Thanks to the Moon Cell, the exploration of these infinite worlds acted much like that of the Undertale Multiverse. Infinite with all kinds of possibilities. Because of this. everything had happened, but never will at the same time. The contradictories it's made fix themselves, making sure to not collapse into itself and cause a multiversal scale black hole.

In the end, the one with the densest infinity was the Fate/Grand Order variant of itself. With so many OCs and possibilities (and hentai cause of degenerates), it lay the grounds of many stories to be made, only seen by so few, yet so many.

The task was for Gudako or Gudao to save humanity from going extinct, gathering the lesser grails and bringing them together to correct the altered history. But Olga was almost always Destined to die. But something changed.

Instead of Olga going into "CHALDEA", when she touched it, the black hole that was "CHALDEA" instead started to change, reacting violently, something happening to it because of a collapse in another black hole that linked itself to it as a sort of gateway. This resulted in not only Olga being saved from her Destiny, but for someone to come out of it, someone that wasn't supposed to exist in their Multiverse.

With this new change, Fate is now different for all parties,