Limbo

Twilight's transformation into a princess has gone horribly wrong, and she's trapped inside the confines of her own mind. There is only one pony kind enough to be able to break through that barrier and save her.

Set just before the end of 'Magical Mystery Cure', before Twilight makes her transition from unicorn to alicorn.

Twilight is stuck in eternal darkness, and only Fluttershy is able to save her.