The War of the Beast was an apocalyptic ork invasion that struck the Imperium roughly a century after the Horus Heresy, threatening to finish what the traitors had begun. It took its name from the greenskin warlord known only as The Beast, a leader of such size and cunning that his coming heralded a WAAAGH! greater than any the young Imperium had ever faced.
The orks came not in raiding bands but in world-sized fleets, and their assault reached its terrifying apex when an ork-built attack moon appeared in orbit over Terra itself. The Space Marines, still reeling from the losses of the Heresy, were pushed to the brink of destruction, and the Imperial Fists were all but annihilated in the fighting. The Imperium's leadership, fractured and complacent, proved dangerously slow to grasp the scale of the danger.
Survival demanded desperate measures and terrible sacrifice, including the raising of new warriors and the forging of uneasy alliances. Though the Beast was ultimately brought down and his WAAAGH! shattered, the cost was staggering, and the war exposed how fragile the post-Heresy Imperium truly was. Its lessons reshaped Imperial institutions for centuries to come. For a deeper account, see the War of the Beast.