Hive Fleet Behemoth was the first Tyranid Hive Fleet encountered by the Imperium and it had a devastating effect on the Imperium when it arrived.
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Hive Fleet Behemoth was the first Tyranid Hive Fleet encountered by the Imperium, and was also the first contact the Imperium had with the Tyranids as a race. Behemoth made a juggernaut rush into the Galaxy from the Eastern Fringe and proceeded into Ultramar, resulting in what became known as the First Tyrannic War. It was eventually destroyed at the Battle for Macragge in 745.M41 or 746.M41 depending on the source. Early warning signs of the apocalyptic threat of Hive Fleet Behemoth came from an Explorator census of the planetary Systems on the Eastern Fringe surrounding Tyran undertaken in the preceding decade before Behemoth’s invasion. Planets known to have been full of life were discovered reduced to lifeless, barren rock without even any living bacteria.
These mysterious new Dead Worlds were reported to the Explorator General but nothing came of it. The outpost on Tyran also filed its own reports of discovered dead worlds, but no help was sent. Hive Fleet Behemoth attacked from the galactic south-east of the Eastern Fringe without warning and ploughed wildly into the galaxy, ruining hundreds of systems and devouring everything in its path. Compared to later Hive Fleets which adopted different strategies, Behemoth showed no subtlety in its berserker advance. Eventually its onslaught used more biomass than it could replace and it all but consumed itself in the process of breaking the Imperial defence at the Battle for Macragge.
The first documented attack was upon the outpost of Tyran in 745.M41. Hive Fleet Behemoth overran the isolated Adeptus Mechanicus Explorator Base with overwhelming force, and stripped the world of all life. A data-codex recording the invasion was stored deep below the base’s command bunker and was recovered almost a year later by Inquisitor Kryptman, a far-sighted Inquisitor who had been dispatched to investigate the reports of unexplained dead worlds. Analyzing the codex, Kryptman realized the grave threat that was entering the galaxy and became determined to warn the Imperium.
Being the first time the Imperium had encountered a new alien race, they were named after the planet Tyran. The System of Thandros contained a Telepathica Matrix orbiting Thandros I, which boosted astropathic signals and allowed intragalactic communications. Kryptman, after discovering the fate of Tyran, attempted to send a message via this Telepathica Matrix. But he couldn’t get through. In desperation he ventured to Thandros and found it destroyed. Kryptman managed to salvage the Matrix so it could project the data-codex along with his own report. The Ordo Xenos replied with orders to travel to Macragge and assist in mounting a defence against the alien fleet. The Ordo Xenos, as dictated by Imperial tradition, also named the invading alien fleet after an ancient and forbidding legend – Behemoth.
Solemnace is a Necron Tomb World that was in the path of two minor tendrils. However they made major course corrections to avoid conflict with the Necrons, and eventually the tendrils lapsed into hibernation after finding themselves unable to pinpoint any nutrient-rich prey worlds. Prandium, known as the “jewel in Ultramar’s crown”, was as easy conquest for Behemoth. The luscious Agri-World was stripped of all life. The Tyranids were so thorough that even Prandium’s atmosphere was consumed for what ever biomass it contained. The Ultramarines, led by Marneus Calgar, mounted a defense of the planet, but had to sacrifice Prandium. For a week Marneus Calgar went without food or water while meditating on the matter. When he came out of his trance-like state, he told the Ultramarines that they had been guilty of the sin of pride and that they would have to adapt and develop new strategies and tactics, even weapons. Hit and run raids were abandoned in place of concentrated defenses and static firepower. This meant that the Ultramarines had to choose a place to defend, and they chose their fortress monastery on Macragge.
Once defeated at Macragge, Behemoth’s threat to the galaxy had ended. The Hive Mind had sampled a significant amount of new biomass and gained experience of how the galaxy’s defenders tactically operate. It would also be years before its creatures were scoured from Macragge and before the Ultramarines and Ultramar could replace all their losses. After the Battle of Macragge, the Hive Fleet splintered into smaller fragments that scattered in all directions. Since, given time to absorb fresh biomass, these splinters could grow large, the Imperium had to deal with plenty of fresh Tyranid threats during the following decades.
Nevertheless, the Imperium allowed itself to think that the worst of the Tyranid menace was over, until it became clear during the last century of M41 that the scattered Tyranid forces being discovered on worlds in the Eastern Fringe were the precursors of two new Hive Fleets, Kraken and Leviathan. One fragment of Hive Fleet Behemoth made its way into the Mechanicum-held Daugel Helix. Here the Mechanicus was able to use knowledge from the Tyran data-codex to give their Skitarii warriors a fighting chance. The Cohor commander Alpha 9-Thyrrc turned the tide by saturating a stratum of the atmosphere with gas from the planet’s Promethium refineries. After his Onager’s Neutron Laser sets the sky ablaze, each new wave of Tyranid spores are set aflame and incinerate those Xenos planetside instead of reinforcing them. However this tactic is stored in a data-codex, brought back to the Forge World of Accatran and promptly forgotten.