The Third War for Armageddon was the long-promised return of Ghazghkull Thraka, and in its sheer scale it dwarfed everything that had come before it. This time the warlord came not as a mere raider testing the defences but as a would-be conqueror, having spent long decades gathering a WAAAGH! of truly unfathomable size, and he made no secret at all of his intent to make Armageddon into his greatest and bloodiest battlefield.
The greenskin fleets darkened the very skies and hurled asteroid roks and boarding craft against the world's straining defences, and the invasion swiftly became the largest Ork war the Imperium had been forced to face in many long generations. The Astra Militarum, Space Marines drawn from more than a dozen Chapters, towering Titans and the aged but utterly unbroken Commissar Yarrick were all fed together into the meat-grinder to hold the vital hives.
Ghazghkull, cunning as well as monstrously savage, deliberately prolonged the war rather than seeking any swift and decisive victory, for to the orks the endless fighting itself was always the true prize to be won. The ash wastes became a boundless killing ground without horizon, the hive cities mere islands in a heaving sea of green, and the casualties on both sides mounted into figures that utterly beggared comprehension.
There was in the end no clean or satisfying conclusion. The Imperium held Armageddon, but only just, and Ghazghkull withdrew in his own good time having grown yet stronger and more legendary for all the slaughter. The war simmered on for years afterward and left the world a byword for grinding, unwinnable conflict. The full account is told in the chronicle of the Third War for Armageddon.