A Cozy Glow-hating brony is fooled by the nefarious filly herself into switching bodies, now he's the one locked in Tartarus while she's causing havoc in his world. Uh-oh. Also, nopony believes him... because he's Cozy Glow. Makes perfect sense.
A struggling young adult finds herself in Tartarus as the nefarious Cozy Glow. If she wants to go out and freely explore all of Equestria with her villainous buddies... she was first going to have to change this evil filly's future.
After 1000 years of tyranny and despair, one mare attempts to unleash ultimate evil, in a desperate bid to save her people and lead them to a brighter future. What she gets is not quite as advertised on the box.