Pinkie Pie is thinking about asking out Percy as a bit of a cheer up after the events of the third ending in the prequel. But her plan is put on hold when Lady of Chaos Eris, resurrected Timothy, and warlock ram Screwtape blackmail her into turning "this jolly land a place they long to leave" via Screwtape's library.
But when an unexpected ally sends Thomas to Tartarus, Twilight follows. Now they must brave the dangers of Equestria's Hell, but at what cost will the ruler let them go? He won't make it easy.

Based (somewhat) off of the Deviant Art comic "The Six Winged Serpent", only cleaner.
Rated T cause that's as close I can get without rating it M. Gore is mentioned, doesn't happen.

Fan made (sort of) song: Welcome to my Library.