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.
- 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.