- You may be looking for the novel.
Even before taking control of history, the Great Houses of those who would soon become the Time Lords sent pioneers to explore "deep time", or "deep-time".
Ancient Gallifreyan Patience's husband, seemingly a version of the Doctor before the "First", was "a pioneer and leader among [the Doctor's] people, one of the first Gallifreyans to enter the Time Vortex after it was discovered". He notably "led an expedition into deep time", returning with "travellers' tales of monsters and lost civilisations". (PROSE: Cold Fusion) As recounted in The Book of the War, prior to the anchoring of the thread, "early deep-time explorations performed by the Great Houses' pioneers" showed that there were "things at work in the formative future, things which simply couldn't be classified or even monitored by the Houses' own technology". This contributed to the Great Houses' decision to become lords of history. (PROSE: The Book of the War)
The Rassilon Imprimatur was responsible for mapping Time Lords' biodata onto the vortex as pure mathematics, (PROSE: Interference) which protected them from deeptime phenomena. When Umbaste dropped his bio-imprimaturs and exposed himself to the caldera, he spent a year in a fugue state.
Having "deep-time memory" meant a person could remember events from past iterations for history, even if they had been overwritten. This was the only way that some agents of the Great Houses could remember the academy on the Homeworld which the Celestis later transformed into Mictlan.
At the start of the War in Heaven, forces of both the Homeworld and the Enemy emerged from deep-time enclaves to initiate their attacks. During the War House Arpexia's specialists created more deep-time strategies and weapons systems than any of the other Houses. (PROSE: The Book of the War)