After she and her friends fall to an unbeatable foe, Twilight accepts the offer of a mysterious creature, promised that she can re-do the events that led up to the encounter.
It's perhaps the worst mistake of her life.
[Soft crossover with Madoka Magica, but does not require knowledge of the source material to be understood. Does contain spoilers for said series. My personal attempt at speedwriting a multi-chapter work in one night, completed in 5-6 hours.]