The Millenium War was a lengthy war between the Empyrean Lords and the Balaur. After the Balaur rebelled against Aion, their creator, Aion created the Empyrean Lords to defend humanity and his Tower of Eternity.
Eventually, the Empyrean Lords raised an army of Daeva, and ten of the twelve lords went to war with the Balaur. The war would last centuries without any real progress, until both sides agreed to meet at the Tower of Eternity to discuss a peace pact. However, although details are unclear as to what actually happened, the meeting ended with betrayal, and the Balaur stormed the Tower of Eternity. Destroying the tower, they shattered Atreia and were banished from the planet, thus ending the Millenium War.
However, Atreia would not experience peace for long as the Asmodians and the Elyos began to battle for control of the newly ravaged world.