This fanfic is based off of Harlan Ellison's "I have no mouth and I must scream". As well as Hasbro's Movie "Equestria Girls". Please watch Equestria Girls first in case you don't want to read potential spoilers.
Spoiler Alerts for those who have not seen it, basically Sunset Shimmer, the unicorn that previously was the Student of Princess Celestia before Twilight. She got angry with Celestia and fled into the human world. After the events of Equestria Girls, Sunset Shimmer seems to have been rehabilitated by the influence of the Elements of Harmony. But what if Sunset Shimmer returned to Equestria alongside Twilight Sparkle and Spike. A bigger question is what would have happened if Princess Celestia never forgave Sunset for the tension between them. And we all know that Princess Celestia isn't good about dealing with those she hates. After all she did banish her sister to the moon.
My name is Sunset Shimmer. I was a former student of Princess Celestia long before Twilight Sparkle. I had become impatient with my studies and rebelled against the Princess and fled into the human world. I thought time would have made a difference in our relationship, that she had finally relented over my actions. If I had thought that Princess Celestia had been angry at me, I was wrong. So dead wrong. She was furious. It didn't even come close to the hate she now unleashed upon me. She made certain I would suffer eternally and could not do myself in or flee back to the Human world. But she left my mind intact.
It has been hundreds of years possibly since I've returned to Equestria. Princess Celestia has resorted to altering my mind, accelerating and decelerating time as she saw fit. Making seconds seem like months and months seem like hundreds of years. I don't know. Princess Celestia had altered me worst than I could have ever imagined, but she still wishes to hold on to me. A sort of example to all who would oppose her. She doesn't want me to smash my skull, nor hold my breathe until I faint. Or somehow cut my own throat with a piece of metal. However, Celestia had been kind enough to grant me a mirror so I may describe myself to you: I have been altered into a great soft jellylike thing. My body is smoothly rounded, with pulsing white holes filled by fog where my eyes used to be. Rubbery appendages that were at one point my legs now bulks down into rubbery appendages of soft slippery matter. Blotches of diseased, evil grey come and go on my surface as though light is beamed from within. I shamble about, a thing that could never be recognized as a pony.
Princess Celestia has taken my mouth, AND I MUST SCREAM!