The Shadow Crusade was the twin blade to the trap laid at Calth, a campaign of deliberate devastation loosed upon Roboute Guilliman's realm. Where the ambush of Calth had struck the heart, the crusade that followed sought to burn everything around it, ensuring the Ultramarines could never march to their Emperor's aid.
The Word Bearers under Lorgar and the World Eaters under Angron descended upon the Five Hundred Worlds like a plague of fire. Lorgar sought dark purpose in the ruin, guiding his legionaries toward blasphemous rites, while Angron simply revelled in the killing, his Butcher's Nails driving him from massacre to massacre. World after world was put to the torch, populations enslaved or slaughtered, the great civilisation of Ultramar left reeling.
The strategy was cruelly effective. Guilliman, master of order and logistics, found himself forced to fight fires across a hundred fronts, unable to concentrate his shattered Legion for a march on Terra. By the time Ultramar was stabilised, the crusade had achieved its aim: the most numerous of the loyal Legions was held far from the decisive battle. The scars the Shadow Crusade carved into Ultramar would never fully heal.