In My Little Pony Friendship Is Magic, Tartarus is a location first mentioned in the episode "It's About Time".
Tartarus is a place where evil creatures are imprisoned under the guard of the three-headed dog, Cerberus.
Tartarus has finally been shown in the season 4 finale, "Twilight's Kingdom". It seems to be an enormous dark cave, with many jagged rock peaks. Prisoners reside on small platforms on top of said peaks. Tirek was sealed in Tartarus a long time ago, but apparently escaped when Cerberus left his post in "It's About Time", only to be imprisoned again thanks to Rainbow Power at the end of the episode.
Trivia
- The name and idea of Tartarus comes from Greek mythology, where it is described as a deep dark pit below the underworld where the souls of the wicked receive punishment, it also serves as a prison for the Titans. The Friendship Is Magic version appears to be simply a very sinister dungeon. But not close to Hell (religion)
- Lord Arcanon took over Tartarus and made it the headqurters for Demon Council of Darkness.