Eternal War

The Eternal War was a protracted and extremely bloody conflict fought against the Yssgaroth early in the history of the Time Lords.

History
Because the story of the Eternal War was never recorded in the Matrix, there were varying accounts of its start. (PROSE: The Pit)

According to a transmission which Samantha Jones received on Anathema (and which may have been influenced by Faction Paradox propaganda), the war began when Rassilon first opened a black hole in an attempt to draw power from other planes of existence. The black hole reached through to an inhabited plane, and monsters with great leathery wings and batlike heads swarmed through the opening. (PROSE: Interference)

According to The Book of the War, the Yssgaroth escaped into Spiral Politic during the anchoring of the thread. They destroyed the machinery responsible for the anchoring, creating the caldera. (PROSE: The Book of the War)

Whatever the cause, the ferocity and resilience of the Yssgaroth made their defeat very difficult, even for the Time Lords. Conventional weapons were virtually useless. Rassilon ordered the construction of bowships to exploit their vulnerability to being impaled through the heart, though he may have stolen credit for the from someone else. Nevertheless, hunting down the entire species was a protracted business. (PROSE: Interference) Led by Kopyion Liall a Mahajetsu, the Time Lords pushed the Yssgaroth back into their own universe over a thousand years, though billions of people were killed and whole star systems were destroyed. (PROSE: The Pit) Eventually, all but one — the swarm leader — were found and destroyed. (TV: State of Decay, Interference)

The node points in the metastructure of history that were outlets to the Yssgaroth's universe were blocked and reinforced by forced-matter shells disguised as planets. (PROSE: The Book of the War) These planets included the Althosian system (PROSE: The Pit) and Earth. (PROSE: Interference, Head of State)

Legacy
The details of the Eternity War were never recorded in the Matrix, and it became a legend throughout the universe. (PROSE: The Pit)

Vampires were viewed as extremely scary by Gallifreyans. (PROSE: World Game)

General Mirraflex, founder of House Mirraflex, achieved notability during the war. (PROSE: The Book of the War)

Rassilon rose to power during the war. (PROSE: The Pit) During Rassilon's reign as Lord President, his throne was made of the bones of giant bats. (PROSE: Christmas on a Rational Planet)