- You may be looking for the Cataclysm.
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)
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 Rivera Manuscript detailed a renegade's praxis-induced vision of the Enemy's first attack on the Homeworld. In it, 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)
Several decades before the War, the War King came to his people after experiencing a vision of the future War and prepared 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 into the War, the 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 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)