War in Heaven

The War in Heaven was an apocalyptic conflict fought between the Precursors and the Reapers which resulted in the total extinction of the Precursors and their dreaded foe.

Overview
In the beginning, there was one—the Primordial, an ancient being older than the Precursors themselves. They were the First Precursor, and as They sang his music, the universe exploded into existence. Over billions of years, the Precursors gave created life across the cosmos and constructed monolithic megastructures that stood the test of time.

However, sometime in the distant past, far before the Illyrians first took flight in their craftships, a mysterious threat known as the Reapers emerged from the darkest corners of the universe. They waged a bloody war against the Precursors throughout all of the universe, so much so that they breached the Aether in order to exterminate the scattered pockets of the Precursors. As their numbers dwindled, the Precursors turned to more desperate solutions, amassing vast quantities of resources towards the construction of countless superweapons that were capable of stopping the Reapers. In the Book of Sorrows, it faintly describes the events that preceded before the Precursors ultimate annihilation. They were constructing a number of superweapons that were capable of exterminating the Reapers, but what happened to those weapons or the mysterious disappearance of the Reapers remains unknown.

Prophecy
In the last surat of the Book of Sorrows, it stipulates a prophecy—that one day, when the life of the firstborn was snuffed by adamantium, a great reckoning will be unleashed upon the universe. The Reapers, or whatever is left of them, will return and they will return to the galaxy with a great flood that would sweep across the cosmos until all of life is extinguished.