Dark Places of the Inside

The Dark Places of the Inside was the dimension to which the Federator banished the Mara from Manussa, thereby ending the evil of the Sumaran Empire. (TV: Snakedance) The Fifth Doctor suggested it could be thought of as "Inner Space," (PROSE: Kinda) with one account suggesting that the Mara still existed in the TARDIS's databanks, as they were so similar to a living mind. (PROSE: Mark of the Medusa)

Tegan entered this place after falling asleep alone beneath some wind chimes on Deva Loka. It was complete darkness, save for a metal structure of some kind, and seemed to be affected by a person's thoughts, such as when Tegan created ten of herself. The/Several Mara manifested itself/themselves as Anatta, Anicca, and Dukkha, the former two believing Tegan to not be real, while the latter tempted her into letting him have her body for a while. (TV: Kinda)

Aliganza Torp was later confronted by the Mara dressed as an undertaker in the Dark Places, which he felt had an oily texture that would react if he tried to move. (PROSE: The Dreaming)

Behind the scenes
Depending on the understanding of how the Mara works, it is often interpreted that the Dark Places of the Inside parallel the real world, with the structure representing the TARDIS, Anatta and Anicca being Nyssa and Adric playing chess, and Dukkha being the Doctor.