The Event

The Event was the prophesied beginning of the War in Heaven in the devastation of Gallifrey by a surprise attack from the Enemy. (PROSE: The Ancestor Cell) However, due to the War King's preparations, the Homeworld was not in fact devastated, (PROSE: The Book of the War) and the War instead began on Dronid in the Cataclysm. (PROSE: Alien Bodies, The Book of the War)

Prediction
Channeling the powers of the Edifice, Compassion once showed the Eighth Doctor a vision of the Event. (PROSE: The Ancestor Cell) "Frightful images of an Enemy first strike on Gallifrey, boiling away the three oceans and levelling the great mountain ranges in the northern hemisphere until the planet burned and roiled and became a cinder."

- The Eighth Doctor's vision of the first attack on Gallifrey.

The Rivera Manuscript detailed a renegade's praxis-induced vision of the Enemy's first attack on the Homeworld. The enemy first attacked the Homeworld's noosphere, telepathically corrupting many members of the Houses and overwhelming them with seizures and madness. The chapterhouses of the Homeworld were then destroyed by black fireballs that materialised out of the sky, described as "supercharged chunks of the causal nexus itself." At this time, a number of stolen timeships returned to the Homeworld, detonating on materialisation. Enemy soldiers resembling Ashla shock-troops materialised out of gouges in the sky, besieging the groups of survivors in their loomsheds for days before plunging into the machinery of the breeding-engines, detonating and leaving the looms intact but mutated. Finally, the Enemy neutralised the Homeworld's power source and the entire Homeworld vanished into cataclysm. (PROSE: The Book of the War)

In the Doctor's vision, the surviving Time Lords decided to break their non-interference policy and retaliate by going back in time and razing the Enemy battle-worlds before the attack on Gallifrey occurred, preventing the Event. The Enemy erased this counterattack from history by detonating a star-killer in the systems where the Gallifreyans had obtained the materials used to construct their fleet at a point before the materials had been mined. These strikes and counterstrikes continued and quickly spread into a full blown War. (PROSE: The Ancestor Cell)

Legacy
Far into the War, at a time when the Time Lords were losing, details of the Event and several other aspects of the start of the War were kept from the Gallifreyan public. (PROSE: The Ancestor Cell)

On account of the mutation of the looms and the use of black fire, (PROSE: The Book of the War) this devastation of Gallifrey may have been the origin of the Kingdom of Beasts, an alter-time realm containing the devastated ruins of the Citadel following the War in Heaven. The Kingdom was watched over by its corrupted Eye of Harmony, the Great Black Eye, and populated by the Mal'akh, (PROSE: The Adventuress of Henrietta Street) whom many speculated were mutated versions of the Great Houses. (PROSE: The Book of the War, Cobweb and Ivory)

Prevention
Several decades before the War, the War King came to his people after an experience with the enemy and began preparing them for the conflict. (PROSE: The Book of the War) When the War began it was not in a surprise attack on Gallifrey, but in a battle on Dronid which the Great Houses were prepared for. There was an Enemy attack on Gallifrey in the early days of the War, (PROSE: Alien Bodies, The Book of the War) but it was nothing like the attack described in the Rivera Manuscript. (PROSE: The Book of the War)

At a time far in the future of the War, Mathara's Faction Paradox warfleet decided to travel back in history to Romana III's Gallifrey shortly before the first Enemy attack. (PROSE: The Gallifrey Chronicles) The Faction planned to take over Gallifrey, prepare for the Event, and launch their own War against the Enemy. The Eighth Doctor decided to use the powers of the Edifice to obliterate Gallifrey before the Enemy could, destroying the Faction and killing almost all of the Time Lords. Compassion, believing (PROSE: The Ancestor Cell) incorrectly (PROSE: Crimes Against History) that the Enemy was the ancestor cells, believed that the destruction of the Time Lords at this time also prevented the enemy from maturing into a threat to the universe; (PROSE: The Ancestor Cell) however, she would go on to fight the enemy many times later in her life. (PROSE: The Book of the War, God Encompasses, AUDIO: In the Year of the Cat, et al.)