The Indomitus Crusade

The Indomitus Crusade was the great counter-stroke that dragged the Imperium back from the very edge of oblivion in the immediate wake of the Great Rift. When the vast warp storm tore the galaxy in two and severed half of it in howling darkness, the resurrected Roboute Guilliman marshalled the single largest Imperial military undertaking since the distant days of the Great Crusade in a bid to hold the dying realm together by main force.

At the crusade's core marched the Primaris Space Marines, the new generation of transhuman warriors ten thousand years in the secret making, unleashed at long last from the hidden vaults of the Adeptus Mechanicus. Alongside vast fleets, Guard armies and even the golden Adeptus Custodes themselves, they were forged into hundreds of battle groups and flung outward from Terra to relieve besieged and forgotten worlds across the whole span of the galaxy.

For roughly a full century Guilliman drove his crusade through the fires and madness of the Great Rift, fighting resurgent Chaos, rampaging xenos and the total collapse of Imperial order on a thousand separate fronts at once. World after world was reconquered or reinforced, shattered defence lines were painstakingly rebuilt, and fragile hope was rekindled in dark places that had long known nothing but despair. Fresh fleets and newly founded Chapters were raised in the crusade's very wake to garrison and hold everything that had been so dearly won.

The crusade could not undo the catastrophe that had birthed it, but it steadied a foundering empire and bought it the precious strength to fight on. When Guilliman at last returned to Terra, the Imperium he had saved was harder, warier and reforged in blood for a far darker age. The full account is told in the chronicle of the Indomitus Crusade.