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. (PROSE: The Book of the War) By one account, 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) Devonire could see the caldera on a balcony from his home. (PROSE: PROSE: The Book of the War'')

By one account, 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) this matched an account showing that the Eye of Harmony, the singularity created by Rassilon (TV: The Five Doctors) which served as the "hitching post" of the Web of Time itself, (AUDIO: Neverland) became buried beneath the Panopticon, its location forgotten by all but a few Time Lords, by the Fourth Doctor's lifetime. (TV: The Deadly Assassin) By another account, the caldera remained exposed in the millennia after its creation, used as a point where all Time Lords would walk to as children. (PROSE: Tempered)

Origins
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 to wasn't really Allah. (PROSE: Head of State)

Modern use
President Umbaste went into a coma after staring into the caldera. (PROSE: The Book of the War)

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

Other information
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.