Caldera

The caldera was a location on the Homeworld that acted as a node point where all lines of historical influence converged and one could stare directly into the heart of time, on the sight of the machine-heart (PROSE: The Book of the War) whose centre had been the obelisk channeling the power of the Eye of Harmony. (COMIC: The Final Chapter) The Untempered Schism was located at the heart of the caldera. (PROSE: Tempered)

It was created during the anchoring of the thread and was where history was born and the Yssgaroth entered reality. (PROSE: The Book of the War)

Location
The caldera was located near the Capitol, by one account as a crater in the mountainside of Solace and Solitude. (PROSE: Tempered) The caldera was created at the site of the "machine-heart" used in central ceremony of the anchoring of the thread, (PROSE: The Book of the War) shown in one account to have taken place out in the open, with the obelisk channeling the power of the Eye of Harmony standing in the centre of a plain surrounded by distant mountains. (COMIC: The Final Chapter)

The caldera was covered over in later eras, becoming a "womb" facility which was the only place on the planet in which timeships could be made; (PROSE: The Book of the War) the obelisk was buried directly beneath the Panopticon, its location forgotten by all but a few Time Lords, by the Fourth Doctor's lifetime. When he rediscovered the secret, was able to raise the obelisk, though the Doctor prevented him from unleashing its power. (TV: The Deadly Assassin) Thereafter, Devonire could see the caldera on a balcony from his home. (PROSE: The Book of the War)

Origins
The caldera was created on the day of the anchoring of the thread. In a special ceremony, "elite representatives" of each of the Ruling Houses gathered (PROSE: The Book of the War) with Rassilon (COMIC: The Final Chapter) at the "machine-heart" on the Homeworld. (PROSE: The Book of the War) Rassilon used the Great Key of Rassilon to activate the Eye of Harmony, effecting the anchoring. (COMIC: The Final Chapter)

However, the moment of strain allowed a "primal manifestation" of the Yssgaroth to break through from another universe. As it broke through, the entity destroyed the machinery and created a crater that was also a wound in space-time itself: the caldera. (PROSE: The Book of the War)

According to The Thousand and Second Night, the "cauldron of creation" was made when the world of the djinn was destroyed, and Allah placed within it "an infinite blackness". At the end of days, the tale said that Allah would dissolve the world in the cauldron and create the new world. However, Iblis, the leader of the djinn, wanted the cauldron so he could have Allah's power, so he convinced the djinn to help him steal the cauldron from Jannat and bring it to Jahannam. Allah sent his angels to defend the cauldron, and many were killed by the djinn.

However, there was a person who was neither an angel nor a djinn, whose name was known only to Allah. He advised Allah to hide the cauldron from the djinn and protect it from them, so Allah formed the Earth around the cauldron and placed Man on Earth to guard it. Now unable to reach the cauldron, the djinn mixed with human women and created the Mal'akh, who lay in wait until the angels returned to take the cauldron back to Jannat. The Shift hinted that it used to be the person with the unknown name, and that the person it was speaking about wasn't really Allah. (PROSE: Head of State)

Uses of the Untempered Schism
The heart of the caldera came to be known as the Untempered Schism. (PROSE: Tempered) When the Tenth Doctor's TARDIS brought him to ancient Gallifrey, allegedly "billions of years" before his time, he and Cindy Wu witnessed young Gallifreyans being forced into the schism by a group of Time Sentinels. (COMIC: Old Girl) During at least one ceremony during the Dark Times, a group of students were thrown into the Schism. Their physical forms were splintered by unprotected travel through the Time Vortex. (COMIC: The Wishing Well Witch)

In later eras, the Schism would come to be used as part of an initiation ritual for children of Gallifrey, who were taken before the Schism at the age of eight to stare at the "raw power" of time and space through the gap in the fabric of reality. The young Doctor and young Master both underwent this ritual. (TV: The Sound of Drums)

Oblivion and rediscovery
Eventually, the citadel was built up over the site of the caldera, which became a "womb"-like crypt of sorts which was the only place where new timeships could be born. (PROSE: The Book of the War) The obelisk channeling the power of the Eye was located directly beneath the Panopticon, with a mechanism activated by the Rod of Rassilon allowing it to be raised into the centre of the Panopticon if necessary. Its location was forgotten, and its sheer existence became a myth.

Eventually, its location was rediscovered by the Master, now in bereft of regenerations. He arranged the assassination of the President, stole the Rod and raised the obelisk, intending to release the power of the Eye to grant himself new regenerations, destroy Gallifrey, and gain "unlimited power" over the physical universe. He was prevented by the Fourth Doctor. (TV: The Deadly Assassin)

Renewed use
A short while after the murder of the President, the Grandfather began to lay the groundwork for the foundation of House Paradox. (PROSE: Crimes Against History) His Godfather-lieutenants travelled through the caldera to access an "abomination's graveyard" from which they sourced the skeletons of what appeared to be members of their own kind corrupted by the Yssgaroth taint, which they claimed were from an alternate timeline where the Homeworld had lost the war against the Yssgaroth. (PROSE: The Book of the War)

During the War in Heaven
President Umbaste went into a coma after staring into the caldera. (PROSE: The Book of the War)

In art and culture
In tapestries on the Homeworld, the caldera was depicted as a black disk with Yssgaroth swarming out. (PROSE: Against Nature)

Imitations
In the 51st century, humanity engineered an artificial, sentient sun as a power source that was also a temporal singularity, similar to the Time Lords' Eye of Harmony. It was known as Crivello's Cauldron and temporarily became the Black Sun. (COMIC: Fire and Brimstone)

Behind the scenes
The caldera's description and function is similar to how the Eye of Harmony is presented in non-televised media, specifically The Adventuress of Henrietta Street. The audio story Neverland asserted that the Eye of Harmony was the "hitching post" of chronology, the central point from where the Web of Time was anchored. The description of the caldera in The Book of the War being "covered over" in later eras of Gallifreyan history seemingly alludes to the plot point in The Deadly Assassin of the Eye's location having become an obscure secret after the Panopticon floor itself was built over it. This does not necessarily imply that The Book of the War endorsed the popular conflation of the Eye with Omega's engineered sun as documented in The Three Doctors, with the "engineered sun" powering the Homeworld being alluded to in the Book separately from the caldera.

In an unexpected twist, however, the caldera was similar to the Untempered Schism later introduced on television in The Sound of Drums, being a gap in the fabric of reality on Gallifrey of great mythic significance into which characters sometimes looked, or even walked, to scry the timelines, despite the experience driving some Time Lords insane. In a rare case of acknowledgement of Faction Paradox elements in material stemming from the BBC Wales version of Doctor Who, Dave Rudden's short story Tempered later alleged that the Schism was located at the heart of the caldera.