Hive Fleet Behemoth was the first major Tyranid Hive Fleet encountered and defeated by the Imperium of Man. The ensuing conflict became known as the First Tyrannic War. The Hive Fleet was eventually destroyed in 745.M41 by the combined might of the Ultramarines, the White Scars, and the Imperial Navy.
Coming from the Eastern Fringes of the galaxy, Hive Fleet Behemoth invaded the Imperium of Man's Ultima Segmentum. It is likely that the Tyranid Hive Mind was drawn to the Milky Way Galaxy by either the beacon of the Astronomican in the Warp or the potent psychic presence of the Emperor himself. Thankfully, the Ultramarines stood in its way.
The first documented attack of a Tyranid Hive Fleet upon the Imperium was at the outpost of Tyran. The Tyranids overran this isolated outpost of the Adeptus Mechanicus with overwhelming force, and stripped the world of all life. By the time Inquisitor Kryptman of the Ordo Xenos arrived, to investigate the strange planet-stripping phenomenon, it was already too late.
However, the Adepts of the Adeptus Mechanicus who accompanied Kryptman managed to gather vital information. Before being overrun, Magos Techpriest Varnak had gathered information on the alien attackers into a data codex and hid it in a bore shaft, some 3000 meters below the planet's surface, hoping that it would be recovered later by the Imperium. Analyzing the recovered information, Inquisitor Kryptman managed to warn the Imperium. Honouring an ancient tradition of naming invading alien forces by giving them an appellation drawn from the great beasts of ancient Terran myth, this first Tyranid Hive Fleet was designated as Behemoth.
The Ultramarines were quick to react, and the newly appointed Grand Master of the Chapter, Marneus Calgar, deployed the Ultramarine fleet to stop the Tyranid Hive Fleet at the garden world of Prandium - the jewel of Ultramar, with gardens and forests of such amazing beauty and genetic craftsmanship that it was considered one of the Wonders of the Imperium - a world that the Ultramarines were duty-bound to protect.
It was a slaughter. The tactics and strategies decreed by the Codex Astartes, a tome of knowledge created by the Ultramarine Primarch Roboute Guilliman to govern all aspects of Imperial warfare, did not succeed against the Tyranids. The harrying strategies of the Ultramarines could not inflict enough damage against the Tyranids - and the close-range passes of Thunderhawks and other Imperial aircraft and vehicles for optimal firepower favored the Tyranids' mode of attack even more, since their spore cysts and clawed tentacles would burst or tear open many of the ships that went in on the attack.
The Ultramarine fleet had to sacrifice Prandium to consumption by the Hive Fleet. 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. The Codex Astartes was written for the Space Marines, the Space Marines were not created for the Codex Astartes. They would have to adapt and develop new strategies and tactics, even weapons, if they were to be successful against the Tyranid menace. 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 their homeworld of Macragge, the very heart of Ultramar.
The fighting was intense, and every Marine of the Chapter was deployed to defend the planet. Millions of Mycetic Spores were destroyed by the planetary defenses, yet millions more got through and reached the surface of the world. Around the frozen northern laser defense batteries, Tyranid bodies stacked 10 deep. So corpse-covered was the ground, in fact, that one walked on them, and not on the frozen soil. The entire veteran Ultramarine First Company was wiped out - an enormous loss that the Chapter could ill-afford.
In space, Inquisitor Kryptman was able to discover a weakness of the Tyranid Fleet: one of the Tyranids' major bioships was destroyed and the surrounding bioships became unable to coordinate before the arrival of a similar vessel. Striking as hard as possible, and losing many ships in the process, the Space Marines were able to destroy the Tyranid Fleet.
Unknown to the Ultramarines, Hive Fleet Behemoth was in reality separated into two forces, leapfrogging each other as they gorged on the worlds they invaded. The Ultramarines' victory at Macragge had only destroyed half of the enemy's strength. However, the arrival of Battleforce Tempestus, the major Imperial fleet of the entire Segmentum Tempestus, brought much-needed reinforcements. Still, Dominus Astra, flagship of Battleforce Tempestus, was sacrificed in a collision with the bulk of the Tyranid fleet while its Warp engines were set to overload. This sacrifice allowed the Imperium to finally claim victory over the Tyranid threat, at least for a short time. The Tyrannic Wars had only just begun.
It took a full Terran century for the Ultramarines to recover their full strength, and only now is the Chapter Banner of Ultramar allowed to be carried into battle forcefully once more by a full-sized Ultramarine Chapter.
Legacy of Behemoth
The Ultramarines' realm of Ultramar was deeply penetrated by the first Tyranid force to make contact with the Imperium of Man, although the region lost only one of its precious worlds, Prandium. Despite only losing one major world to the rapacious xenos, many minor worlds of the realm 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 much of the rest of the Chapter's forces, and the Imperial Navy also took a sizeable amount of time to recover from its losses. Also, the Tyranids left behind a deadly biological weapon 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, as the next threat to the Imperium was the arrival of Hive Fleet Kraken and the start of the Second Tyrannic War.
- Warriors of Ultramar by Graham McNeill
- Warhammer 40,000 Chapter Approved - The Book of the Astronomican