Hive Fleet Behemoth

Hive Fleet Behemoth was the first Tyranid Hive Fleet to enter the galaxy. It was destroyed during the First Tyrannic War of 745-746.M41. The discovery of the Tyranids, as heralded by Behemoth, was one of the first major events in the Time of Ending.

For nearly a decade prior to their confirmation, the Explorator Fleets were aware that something was happening out in the Eastern Fringe, as worlds which had previously been confirmed to support life had been reduced to Dead Worlds, even to microscopic life. Inquisitor Kryptman of the Ordo Xenos was dispatched to investigate. When he reached the Explorator Base on the planet Tyran, he discovered a lifeless rock, but a single data-record was left detailing what happened, leading to the designation of the Tyranids (Tyran ID). Kryptman immediately declared them xenos horrificus, an alien life form so destructive that extinction was the only option. He then discovered that they were heading straight for Ultramar.

Upon learning this, the Imperium of Man suspended the Damocles Crusade that had been declared against the Tau Empire, both to reallocate resources and because the High Lords of Terra had decided that the Tau would serve as a nice buffer between them and this new threat. Marneus Calgar, the Chapter Master of the Ultramarines, took most of the Chapter to the Ultramar world of Prandium. However, the Tyranids were able to overwhelm the Ultramarines and force them to abandon Prandium. Calgar spent a week fasting and declared that the Ultramarines had suffered the sin of pride by allowing themselves to become complacent (bear in mind this was years before our spiritual liege was working at Games Workshop). Falling back to Macragge, the Ultramarines fortified their chapter homeworld, but the Tyranids responded by unleashing the Swarmlord. Despite the force feeding best efforts of his Honour Guard (including the sacrifice of their commander), Calgar was so grievously wounded that he lost both his arms and legs, one eye, and his stomach, and had to retreat to orbit, leaving the northern polar fortress with only the First Company to defend it. Fortunately, Imperial Navy reinforcements from Segmentum Tempestus (yes, the Tyranids were so bad they needed reinforcements from another Segmentum) finally arrived. The battle only turned in the Imperium's favor when the Emperor-class Battleship Dominus Astra detonated its warp-drive, dragging most of Hive Fleet Behemoth into the Warp and letting them meet the Chaos Gods. Returning to Macragge, the Ultramarines Third Company scoured the northern fortress, where they discovered First Captain Saul Invictus and the entire First Company had been killed to a man. Only one member of the First Company, Battle-Brother Altarion, had survived the battle, and he was so messed up that he had to be put into a Dreadnought, which of course invites philosophical debate on whether he actually died or not, but that's for another day.

While the Imperium had won, the cost of Hive Fleet Behemoth was severe. An entire world of Ultramar was lost and the Ultramarines First Company with it. Captain Severus Agemann of the Second Company was promoted to First Captain of the Ultramarines, while Veteran Sergeant Cato Sicarius was promoted to Captain of the Second Company. The losses the Ultramarines suffered were so great that Dante sent several companies of the Blood Angels to help the Ultramarines purge the remaining Tyranids from Ultramar. Nearly a century later, the Ultramarines had still not fully recovered from the losses Hive Fleet Behemoth had inflicted, but by that point Macragge was fully free of Tyranids. Several ships of Behemoth survived, but without the coordination of the Hive Mind they broke apart into smaller Hive Fleets. The Swarmlord was officially listed on the rolls of the fallen, but its corpse was never located.