The hive fleets so far introduced by Games Workshop include Hive Fleet Behemoth, Hive Fleet Kraken, and Hive Fleet Leviathan. It is noted that these names are those ascribed to the Tyranid incursions by the scholars of the Imperium, rather than the Tyranid civilization themselves. Indeed, there is no evidence shown in the fiction that Tyranids even
have language or civilization as understood by the protagonist civilizations native to the Milky Way, but the Tyranids appear only able to be communicated to by the Hive Fleet leader and become ineffective and uncoordinated if it is destroyed. Hive fleet Behemoth invaded in a giant swarm of close to 1000 ships, and reached as far into Imperial Space as Macragge, the homeworld of the Ultramarines, before it was destroyed. Hive fleet Kraken was a large number of sub-fleets, which were eventually halted at the Battle of Iyanden and the Battle of Ichar IV. Surviving vessels spread out throughout the galaxy, forming the Splinter Fleets. Hive fleet Leviathan was made of a C shape approaching from below the galactic plane, spreading the phenomenon known as the Shadow in the Warp.
Hive Fleet Behemoth
As described in the rule book and the novel Warriors of Ultramar, Tyranids were first encountered by the Imperium in the Eastern Fringe of the Galaxy, near the planet Tyran. Adeptus Mechanicus scientists noticed several worlds that had mysteriously lost their biomass and atmosphere, then the scientists themselves disappeared. Later an Imperial Inquisitor named Kryptmann visited Tyran and found a data file with information on the Tyranids, left behind by the former occupants before their deaths. This is when the Tyranids were first named, receiving their race's (terror-inducing) name.
Despite this, several planets were devoured by the Tyranids, but Kryptmann could not call for help due to the Shadow in the Warp. The Tyranids, now named "Hive Fleet Behemoth" by the Imperium, progressed further into human held areas of space, eventually reaching the Ultramar sector, the realm of the Ultramarines Chapter of the Adeptus Astartes.
Marneus Calgar, Chapter Master of the Ultramarines, recalled all his forces to the Ultramarines' homeworld, Macragge. Assembling a fleet and readying the Ultramarines to face the Tyranid vanguard. The Tyranids encircled and overwhelmed the lush garden world of Prandium - stripping its surface bare of life.
The Tyranids moved to attack Macragge itself. The true power of the Hive fleet was shown as even though a warp vortex removed the hive fleets Synapse control, there was still a titanic struggle and great losses - including the heroic last stand of the Ultramarines veteran First Company, but eventually, the Ultramarines prevailed. The Tyranids threatening Macragge were slain and driven back. Reinforcements arrived...just before the hive fleet's second wave. The allies parted company and renewed their efforts to prevent the Tyranids regrouping. The final action against Hive Fleet Behemoth occurred in orbit of the gas giant planet of Circe. At the height of the battle the flagship Dominus Astra charged straight into the heart of the Hive Fleet and triggered its warp drives, destroying it utterly
Hive Fleet Kraken
The second wave of Tyranids to fight against the Imperium was known as "Hive Fleet Kraken". Its arrival was heralded by a score of genestealer rebellions and insurrections on the Eastern Fringe. Instead of throwing one mass of troops against the human armies, this swarm split into countless smaller fleets, each one enveloping a few planets, resulting in the loss of whole systems before reinforcements could arrive. Although this made each individual assault weaker and made some warp travel and communication possible, Kraken was attacking across a far vaster front than Behemoth. Two Space Marine Chapters based on the Eastern Fringe - the Lamenters and the Scythes of the Emperor - were all but annihilated. A vast portion of Kraken's strength attacked the Eldar Craftworld of Iyanden Although eventually defeated, the Invasion killed four-fifths of the Craftworld's population; a terrible blow to the dwindling Eldar race
As the fleet faced more severe resistance, smaller sub-fleets combined to overcome it, narrowing the range of the Tyranid advance as it pushed further into the Imperium. After the Battle of Iyanden and the Ultramarines successful destruction of the Ichar IV rebellion, the Imperium's psykers were able to predict where the remainder of Kraken's strength was moving . Troops were rushed to Ichar IV ahead of the Tyranids, where Marneus Calgar, Chapter Master of the Ultramarines and victor of the first Tyrannic War, was elected Supreme Commander. Once again, Calgar led the Imperium to victory. Yet this was not the end of Kraken, for after the battle, the surviving vessels divided into a series of 'splinter fleets'. Although tiny by comparison, these continue to threaten isolated and poorly defended worlds, and increase their numbers with every consumed world.
Hive Fleet Leviathan
Five years after the defeat of Hive Fleet Kraken, Lord Inquisitor Kryptman began to notice the tell-tale signs of a Tyranid invasion. Implementing the infamous Kryptman census, the Inquisitor burnt out dozens of astropaths in an attempt to contact every civilised planet on the fringes of the Imperium. Slowly the responses formed a pattern. The ancient Inquisitor charted the path of the Tyranid's latest hive fleet. Codenamed 'Leviathan', the fleet was coming from below the galactic plane, attacking from two wide points. The fleet appeared as a pair of closing jaws, cutting off huge stretches of space from warp travel and astropathy. A combined force of PDF troops and Space Marines fought off against the Tyranids on Tarsis Ultra, but were pushed back and back by the tyranid force, until Kryptman captured a live Lictor. By reverse-engineering a virus, Kryptman, through a Deathwatch kill team led by Ultramarines captain Uriel Ventris broke into the hiveship and injected the virus into the norn queen, which then died and causing all synapse to be lost from the Tyranids. Slowly, but steadily, the Imperial forces retook Taris Ultra, only to find most of the system destroyed. It was a victory, but at a great price. Several systems in the path of the Tyranids had been destroyed, and the other half of Leviathan was crushing those that were before covered by the shadow of the warp.
