The War in Heaven was a cataclysmic conflict fought in the unimaginably distant past, tens of millions of years before the rise of humanity, whose consequences still shape the galaxy in the present age. It was waged between the ancient necrons, then a mortal race called the necrontyr bound to godlike star-devouring entities, and the enigmatic Old Ones who had seeded the galaxy with life.
Seeking to overthrow their masters and cheat death, the necrontyr had bargained away their flesh, transferring their essence into undying metal bodies and enslaving themselves to the hungering star-gods in the process. United under these terrible powers, they turned upon the Old Ones, whose mastery over the warp had allowed them to shape countless species. In answer, the Old Ones created warrior races to fight on their behalf, among them the ancestors of the Aeldari and other psychic peoples.
The war raged across the stars for ages, reshaping the galaxy and ultimately consuming the Old Ones and shattering the necron empire alike. In its aftermath the victorious necrons, betrayed and depleted, entombed themselves in stasis to sleep away the eons, while the wounds torn in reality helped give rise to horrors still felt today. The War in Heaven is the primordial root of much that plagues the galaxy, an ancient enmity whose embers smoulder still. For more, see the War in Heaven.