Hive Fleet Behemoth was the first of three Tyranid Hive Fleets that were to attack the Imperium of Man. It was first sighted in the 745th year of the 41st millenium by an Adeptus Mechanicus Explorator station. The vessel had noticed several planets on the outer fringes of Ultramar that had been mysteriously attacked and consumed spontaneously. The station also noticed that, after the attacks, the planets were completely devoid of life or anything that could support life. They were just giant masses of rock, floating in space. Unfortunately, shortly after making this discovery, the Explorator station was destroyed by the menace.
After these events, an Inquisitor named Kryptman was able to see when and where the aliens were going to next attack. This new knowledge did little to help the defenders of man, however, and the newly-named Tyranids were too powerful for even the Ultramarines to stop. They could only watch and fight hopeless battles trying to defend Ultramar from the unstoppable force, incapable of even defending the precious garden world of Prandium. The invasion of Hive Fleet Behemoth finally culminated on the Ultramarine homeworld of Macragge. The Battle of Macragge was a vicious foray that finally ended in the destruction of the Tyranid invaders, although the Imperial forces also suffered massive casualties. The Imperial Navy lost nearly 200 ships, and the Ultramarines lost its entire First Company along with numerous other casualties. In addition, millions of casualties were inflicted upon the world of Macragge itself.
Legacy of Behemoth
The realm of Ultramar was deeply penetrated by the Tyranid force, although the region lost only one of its precious worlds, Prandium. Despite only losing one major world, many minor worlds were consumed by Hive Fleet Behemoth, and the damage was still quite extensive. It would take the Ultramarines nearly a century to recover from the loss of its entire First Company, and the Imperial Navy also took a sizeable amount of time to recover from its losses. Also, the Tyranids left behind a deadly bio-creature in Ultramar: Old One Eye.
Despite these apparent dangers and losses, the Imperium gained a vast amount of knowledge about the Tyranids. This knowledge helped the defenders of man against future Hive Fleets, which were soon to come, the next one being Hive Fleet Kraken.