Mictlan was the extra-dimensional home of the Time Lord faction known as the Celestis. It served as a metaphysical bomb shelter removed from spacetime. It made its occupants immune to time winds or changes imposed on Gallifrey's past. (PROSE: The Taking of Planet 5) The appearance of this realm varied with the perceptions of its inhabitants. (PROSE: Alien Bodies) It 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 Bone Museum of Mictlan served as a library for agents of the Celestis. It held information on the Doctor. In addition, there was the Plain of Bones located in this realm.
It was destroyed due to the actions of the Hermit, a renegade Celesti and of the Fendahl Predator. (PROSE: The Taking of Planet 5) The Celestis took those close to death to Mictlan. (PROSE: Autumn Mist)