Mount Perdition

 was a mountain on the planet Gallifrey. Its slopes were covered with vast fields of red grass. (TV: The End of Time)

During the Dark Times, in the time of Pythia's Curse, (PROSE: The Infinity Doctors) Mount Perdition featured a tributary which contained that era's Time, which the Stranger tried to turn into a stream. It would soon become the River of Time, connecting to the Sea of Life. (AUDIO: Patience)

In the Old Times, the battle between Presus and Death's Messenger began in a cave and the foot of Mount Perdition. (PROSE: Blind Fury) The Caves of Perdition were guarded by Ansillon, a Time Lord hermit who lived on the mountain. (AUDIO: Blood of the Time Lords)

The Eighth Doctor once told a story about the Gallifreyan Lady Patience seeking the help of the Stranger, a hermit who lived on the slopes of Mount Perdition, as she wanted to have a child, despite Pythia's Curse. (AUDIO: Patience)

Later history
The Master's father owned land on Mount Perdition's slopes. (TV: The End of Time)

During the Last Great Time War, the Celestial Intervention Agency had a base on Perdition. (AUDIO: Darkness and Light) During the early days of the war, many items from public buildings in the Citadel were placed in a facility underneath the mountain for protection. (AUDIO: Havoc)

Behind the scenes
Perdition is described in Christian theology as a state of external punishment and damnation into which a sinful and unpenitent person passes after death.