The Battle of Macragge was the Imperium's first full reckoning with the tyranid menace, a desperate defence of the Ultramarines' homeworld against a devouring alien swarm. It came when the vanguard hive fleet, the first great tendril of the tyranids to reach the galaxy, descended upon the world of Macragge to consume all life upon it.
The Ultramarines and their planetary defenders fought a two-fold war, holding the northern polar fortresses against the tyranid ground assault while their fleet battled the swarm in the void above. The fighting on the surface was savage beyond measure, as endless waves of alien organisms threw themselves against the defenders' lines with mindless, tireless hunger. The war stretched Macragge's forces to breaking, and whole regions were overrun before the tide could be stemmed.
Victory demanded a supreme sacrifice. In orbit, the Ultramarines' First Company, their veteran Terminators, boarded the tyranid hive ship in a suicidal action to destroy it from within, and to a man they perished in the attempt. Their sacrifice broke the fleet and saved the world, but the Chapter was left grievously wounded, an entire company of its finest warriors annihilated. The battle stood as a dire warning of the horror advancing from beyond the galaxy's edge, the first shadow of an extinction-level threat that would only grow.