The Final Battle is when the Dark Forest and their clan recruits will invade the living clans and attack them and take over any cats they don't kill. It is the main conflict in the Omen of the Stars series. It is unknown when this will happen but it must happen sometime in the Last Hope because it is the last book in Omen of the Stars. The three are supposed to stop this battle, or maybe the four, they just recieved a prophecy saying there must be four. Also with the help of Ivypool, who is training with the Dark Forest but is really working against them. Together these cats may be able to stop the Dark Forest from taking over. The Erin Hunters have confirmed that there will be survivors after the Final Battle.
The Final Battle happened in The Last Hope. All of the cats training with the Dark Forest (excluding Breezepelt) didn't fight with them. It left Redwillow, Beetlewhisker, Hollyleaf, Ferncloud, Mousefur and Firestar dead. It destroyed the spirits of Spottedleaf, Shredtail, Brokentail, Hawkfrost and Tigerstar.