Tartarus

Tartarus is first mentioned in It's About Time as a place where ancient, evil creatures are imprisoned, guarded by the three-headed dog Cerberus. The location is mentioned again in Twilight's Kingdom - Part 1, where Lord Tirek is stated to have been confined there for the attempted theft of Equestria's magic, but escaped during the events of the former episode. In Twilight's Kingdom - Part 2, Tartarus makes its first appearance when Tirek conjures a portal to banish Princess Celestia, Princess Luna, and Princess Cadance there in revenge for his own imprisonment; and later when Twilight Sparkle and her friends return Tirek to Tartarus and free the princesses. It is also mentioned on My Little Pony: Friends Forever Issue #14 page 11, in Slice of Lifewhen Sweetie Drops says the bugbear attacking Ponyville originally escaped from there, on My Little Pony: Friendship is Magic Issue #34 page 1, in a flashback in Celestial Advice when Celestia gets worried that Twilight might get pulled into Tartarus if she sends her off to Ponyville, and in My Little Pony: Legends of Magic Issue #11. In School Raze - Part 1, the Mane Six and Spike visit Tartarus when the magic in Equestria starts to disappear and they suspect Tirek to be the cause. In Part 2, the Mane Six and Spike escape from Tartarus only to find out they are too late to stop Cozy Glow. At the end of the episode, as punishment for her actions against Equestria, Cozy is sentenced to imprisonment within Tartarus. In The Beginning of the End - Part 1, Tirek and Cozy are freed from their imprisonment by Grogar when he magically transports them out of their cages. In My Little Pony: Friendship is Magic Issue #82, Tartarus is mentioned and its entrance is seen on page 18. Tartarus is based on the location of the same name in Greek mythology, being a deep, dark pit beneath the underworld where the wicked are condemned to eternal suffering. In It's About Time, Twilight Sparkle irregularly pronounces the name Tartarus as  /tɑrˈtɑrəs/ (tar-TAR-es), while it is pronounced in its traditional form as  /ˈtɑrtərəs/(TAR-tar-us) in later episodes.

In When the Cold Breeze Blows Away, it has been turned into the main base of the Group P365 after they were captured the Cerberus and take over it since it was a place where ancient, evil creatures are imprisoned, guarded

Also,it is where the zombie dogs and types of pony zombies were located and guarded that the Group P365 can create their types of zombies and pony zombies and even an inventions with time machines.

In post-Last Day, it became the main base of the New Group P365 after the rise of the New Group P365.