Mictlan

Mictlan was the extra-dimensional home of the Time Lord faction known as the Celestis, created as a hiding place from the War in Heaven. (PROSE: Alien Bodies, et al.) It was named after the underworld of Mexica culture, Mictlan. (PROSE: Against Nature)

Mictlan was uniquely constructed to be an entire conceptual ecosystem, made only of pure thoughts, ideas, and concepts instead of physical matter. This caused to realm to be similar in nature to conceptual entities. The realm existed in a state of enormous flux that was so corrosive to the rest of the Spiral Politic that it had to be kept on the outer skin of the universe, causing it to become a non-world. (PROSE: The Book of the War) As a result, its occupants were immune to time winds or changes imposed on Gallifrey's past. (PROSE: The Taking of Planet 5)

Mictlan was created by the use of block-transfer engines and computational matrices which also linked all the Lord Celestials who made up the Last Parliament. (PROSE: The Taking of Planet 5)

Appearance & geography
The appearance of Mictlan varied with the perceptions of its inhabitants. (PROSE: Alien Bodies) When Entarodora visited Mictlan, the realm appeared as a grey and black landscape with a glowing sky. (PROSE: The Brakespeare Voyage) One area of the realm was described to have a crystal shard four miles high, the backbone of a Great Vampire, and a night sky. (PROSE: The Taking of Planet 5)

The Bone Museum of Mictlan served as a library for agents of the Celestis. It held information on the Eighth Doctor. In addition, there was the Plain of Bones located in this realm. (PROSE: The Taking of Planet 5) The Celestis took those close to death to Mictlan. (PROSE: Autumn Mist)

The outside of Mictlan was described to be an eventless void with only Black-box TARDISes projected in bubbles of space-time around it to continually scan, record, and check on the realm as a form of protection. (PROSE: The Taking of Planet 5)

The Celestis were considered to be part of Mictlan's own geography. The realm's purpose was to sustain them; their purpose was to have their being with in it. If the realm was threatened, then the very existence of the Celestis could be in danger. (PROSE: The Taking of Planet 5)

History
Mictlan was created by the use of block-transfer engines and computational matrices which also linked all the Lord Celestials who made up the Last Parliament. The realm was described to be a "metaphysical bomb shelter", existing outside space-time so that it and its occupants were theoretically immune to the effects of the War in Heaven. (PROSE: The Taking of Planet 5)

The Eighth Doctor once traveled into Mictlan to confront the Celestis. (PROSE: Alien Bodies)

Entarodora projected her consciousness into Mictlan from The San Grael in order to make a deal with Lord Dervishage involving The Brakespeare. Backup records of this meeting were later given to the Great Houses for use as evidence in Great Houses vs. Scarratt. (PROSE: The Brakespeare Voyage)

Mictlan and the Celestis were destroyed due to the actions of Investigator One, the hermit, and the Fendahl Predator. The realm then was cut out of the universe and expelled into the outer void by a fleet of War TARDISes under the guidance of the Eighth Doctor. (PROSE: The Taking of Planet 5) The only known survivors were a Shift, several servants, and the timeships involved in the attack; together, they colonised a small planet early in the universe. (PROSE: The Book of the War)