The Protomachy, also known as the Great War in Heaven, was the first major conflict in all of reality, and one that has not yet been concluded. It began when a future version of Ayavan wrecked havoc on the Earth from the second era, devouring the timelines from living beings.
One of the most critical points of the war was when the Tower of Dawn was first activated by the Sacred Triumvirate, causing the Great Cataclysm and the creation of Ayavan, who travelled to the past, initiating the war. However, when the Tower was activated fully for the second time, the events surrounding the Great Cataclysm were erased, but the destruction of Atlantis and the creation of Ayavan remained the same.
King of Kings, Ruling Over Rulers (first appearance)