The alicorns have been struck by a fatal illness, and Twilight and her friends must find the cure: a panacea hidden in the deepest reaches of Tartarus. But Tartarus is unnavigable to those who cannot withstand the chaos in its depths, and the spirits of harmony are no exception. To obtain this cure, Twilight and company will need to find ponies who can traverse Tartarus, a task that will take them to the edge of the world and back. Supposedly, these ponies have two things in common: they've all crossed paths with the mane six, and none of them ever learn from their mistakes.
Inspired by, but not quite based on, Braid.
AU tag for deviation from Season 3 canon.