Everypony knows about Tartarus; the mystical underworld where horrible supernatural dangers are kept locked away. But what about an Earth pony pick pocket? A unicorn embezzler. A Pegasus that gets in a few fights too many? An earth pony that makes a candy too sweet to resist? Where do they go?

Blackhorn. For centuries, it is the dungeon where Celestia sends her criminals and misequithropes. The ponies who blow their second chances. The ones who commit horrible crimes too terrible to be allowed to live in general society. Blackhorn is the place where the pony criminal population can be contained, rehabilitated, or forgotten about.

And now, one of the mane six has been sentenced there for the rest of her life.

(The story is just something I was thinking about. It's AU, and I'm hoping to avoid spoilers. Definitely plan to come back to this later. I hope. Really!)