A Fell Mirrored Insanity created by the original creator of Mirrored insanity and all of the text is written by ReveX since you can just check in the description of his videos.
Dustfell Sans has killed almost everyone in the underground, and has now gained a crazy amount of power, he is just about to approach the entrance of The Ruins, when suddenly, a strange figure approaches him from the shadows, this strange figure looks like some kind of monstrosity to Dustfell Sans, so he responds with "What in the 28 hells are you?", but then he looks closer and realizes that this monstrosity looks nearly identical to himself, so he then asks "And why do you look so...Familiar..?", Insanityfell Sans laughs sinisterly, and replies with "Interesting question" "I have a simple answer" "And that answer is..." "I AM YOUR MIRRORED PSYCHOPATHY", and thus, the first battle begins. Dustfell Sans has no clue what this creature is capable of, But he suspects it's dangerous but not as dangerous as it really is, so he goes kind of easy on it, which ends up being a great mistake, Insanityfell Sans ends up killing Dustfell Sans, and Insanityfell Sans ends up victorious...
Insanityfell Sans has just finished Dustfell Sans, however, he's a little disappointed, he expected Dustfell Sans to pull off more of a fight, so he talks to himself in disappointment, he then decides to leave, but just as he's about to, he hears dark laughter coming from the dust pile that happens to be Dustfell Sans, the dust pile reforms itself into Dustfell Sans, and Insanityfell Sans turns around and sees Dustfell Sans in his newer form, giving Insanityfell Sans the death stare, Dustfell Sans then says "You've come a long way from hell..." "It wants you back" "HOW ABOUT I SEND YOU BACK THERE?", and so, the second fight begins, Dustfell Sans is now incredibly pissed off at Insanityfell Sans since he killed him, so now Dustfell Sans isn't holding anything back, he gives Insanityfell Sans everything he's got, and in the end, he kills Insanityfell Sans, Dustfell Sans ends up victorious.
After a long battle, Dustfell Sans is victorious, he has killed Insanityfell Sans, and can now return to what he was originally going to do, Dustfell Sans ends up teleporting past the dust pile, but then realizes, he hasn't gained any EXP, so he turns around and asks himself why he hasn't gained any EXP, suddenly, a red soul pops up above the dust pile, Dustfell Sans is shocked, and suddenly, the dust pile slowly reforms itself into Insanityfell Sans but a very damaged form, now the finale begins. Dustfell Sans and Insanityfell Sans start their long fight, they attack with everything they've got, but in the end, they both end up killing each other, but as they're turning into dust, Dustfell Sans absorbs all the EXP from Insanityfell Sans, and manages to just in time resurrect himself once again, Dustfell Sans ends up being victorious once again. The Outcome Dustfell Sans is fully victorious, but he now knows what kind of threat Insanityfell Sans really was, and Dustfell Sans prays that he doesn't resurrect himself again, and luckily, he doesn't. Dustfell Sans is just getting ready to leave, when suddenly, his soul pops up, and he feels himself getting corrupted by something, he then realizes that when he absorbed Insanityfell Sans' EXP, he also absorbed his soul, which is now corrupting him, Dustfell Sans tries to stop it desperately, but fails, his mind has now been completely corrupted by Insanityfell Sans, and his body has been taken over completely, Dustfell Sans has been eradicated, and InsanityDustFell!Sans is born...
Frisk/Chara wakes up on the flowerbed as they always do in the beginning of Underfell, they get up and enter floweys place, expecting at least *something* to have changed, and they are slightly satisfied, as the first thing they see is Flowey begging them to kill him quickly, he tries to tell them that they can't let Sans kill him, but he gets cut off by a very sudden static screen, and suddenly, Flowey's face is sliced in half, and then the screen rapidly goes static as flowey screams in agony from being tortured to death by none other than Sans, but then he's suddenly gone, and everything is back to normal, Frisk/Chara continue, but just as they land their foot on where Flowey was, they get interrupted by another static screen, as suddenly, Sans appears in front of them, He then starts taunting the victims he killed before arriving here, he then starts mentioning you and how he's gonna endlessly torture you for what you did to him, and thus, the battle begins, Sans stands there menacingly, waiting for you to strike, once you do, the screen becomes static again and then the whole game comes back as if nothing happened, Sans then throw his attacks at you, as this is happening, Sans is just saying random things that makes absolutely no sense, this is obviously due to his insanity, but then in the end of the video, Sans slashes the screen (which is supposed to be the human) and then kills the human, and that's when he says "There Are Fates Worse Than Death", what he means by this is the fact that the human will now go through a much worse fate than death, being stuck in an endless loop of fighting Sans and getting brutally murdered in many ways everytime, and there's no way for the human to stop it, not even resetting will work, as that will only cause the fight to happen faster...