Behemoth
From Lexicanum
Hive Fleet Behemoth was the first major Hive Fleet encountered and defeated by the Imperium. The ensuing war became the known as the first of the Tyrannic Wars. The Hive Fleet was eventually destroyed in 745.M41, by the combined might of the Ultramarines, the White Scars, and the Imperial Navy2.
Coming from the eastern side, Hive Fleet Behemoth invaded the Ultima Segmentum. It is likely that the Tyranids were drawn by either the Astronomican or the psychic presence of the Emperor himself. Thankfully, the Ultramarines stood in its way.
The first documented attack was upon the outpost of Tyran. It overran the isolated outpost of the Adeptus Mechanicus with overwhelming force, and stripped the world of all life. By the time Inquisitor Kryptman arrived, to investigate the strange planet-stripping phenomenon, it was already too late.
However, the Adepts of the Adeptus Mechanicus managed to gather vital information. Before being overrun, Magus Varnak, ranking magus gathered information 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. Honoring an ancient tradition of naming invading alien forces, by giving them a name from great beasts from ancient myth, this Hive Fleet was designated as Behemoth.
The Ultramarines were quick to react, and the newly appointed Grand Master, 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 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 Roboute Guilliman, 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 for optimal firepower favored the Tyranids even more, since their spore cysts and even clawed tentacles would burst or tear open many of the ships that went in on the attack.
The Ultramarine fleet 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. 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 Macragge.
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 planet. Around the frozen northern laser batteries, Tyranids 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 Ultramarine First Company was wiped out - a enormous loss.
In space, Kryptman was able to discover a weakness of the Tyranid fleets: one of the Tyranid's major ships was destroyed and the surrounding ships 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 the Hive Fleet of Behemoth was in reality separated into two forces, leapfrogging each other as they gorged on the worlds they invaded. They only had been able to destroy 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 into the bulk of the Tyranid fleet while its warp engines were set to overload. This sacrifice allowed the Imperium to claim victory.
The Ultramarines recovered their full strength only after 100 years, and only now is the Chapter Banner of Ultramar allowed to be born onto battle again.
