The Battle of Calth was a treacherous surprise attack during the opening act of the Horus Heresy, in which the Word Bearers Legion fell upon the unsuspecting Ultramarines at the industrious world of Calth. Ostensibly gathered for a joint campaign, the two Legions had massed in the system when the traitors struck without warning.
Directed by the fanatical Kor Phaeron and the arch-manipulator Erebus, the Word Bearers sought to annihilate the Ultramarines and delay their response to the wider rebellion. Their ambush caught fleets in orbit and armies on the ground, igniting a furious war across the planet's surface and void. Yet the Ultramarines, though bloodied and taken unawares, refused to break, rallying under their officers to contest every front.
Unable to win outright, the traitors resorted to a monstrous act of spite: they corrupted Calth's own star, bathing the world in lethal radiation. The surviving population was forced to abandon the ravaged surface and retreat into vast underground arcologies, where they would dwell for generations. Though the Ultramarines ultimately drove the Word Bearers off and denied them their victory, Calth was left a poisoned world, its people exiled beneath the earth. The battle marked one of the earliest and most personal wounds of the wider civil war, a betrayal between Legions that had once called each other kin.