Tartarus is a subworld located under Equis, similar to the traditional idea of Hell. Its main entrance is guarded by Cerberus.
Tartarus is present in multiple fics of the Chess Game of the Gods. It acts as an afterlife of sorts for denizens of Equis, as well as serving it's normal purpose in Greek mythology. Similarily, the River Styx is present and posesses many of the same qualities as the original myth, Like Charon being the boatsman and pacts made on the Styx having meaning. It also has many cells and caverns for prisoner's, as well as undead guardians. Many of the locations and entrances were created by the Wardens as a method for storing entities they could not control or reason with. Many of these beings will break out at some point.[UNFINISHED: Needs more exposition] Cerberus guards the entrance to make sure no mortals got in and no souls got out. Occasionally people could get past.