Hive Fleet Behemoth

Tyranids of Hive Fleet Behemoth looking for the all you can eat buffet.

Hive Fleet Behemoth was the first Tyranid Hive Fleet to enter the galaxy. It was destroyed during the First Tyrannic War of 745-746.M41. The discovery of the Tyranids, as heralded by Behemoth, was one of the first major events in the Time of Ending.

Invasion

For nearly a decade prior to their confirmation, the Explorator Fleets were aware that something was happening out in the Eastern Fringe, as worlds which had previously been confirmed to support life had been reduced to Dead Worlds, even to microscopic life. Inquisitor Kryptman of the Ordo Xenos was dispatched to investigate. When he reached the Explorator Base on the planet Tyran, he discovered a lifeless rock, but a single data-record was left detailing what happened, leading to the designation of the Tyranids (Tyran ID). Kryptman immediately declared them xenos horrificus, an alien life form so destructive that extinction was the only option. He then discovered that they were heading straight for Ultramar.

While getting to Ultramar, the Tyranids ate Tyran, ate a moon-sized chunk of biomass the Imperium held for 9000 years at Occulus, Fought the Star Krumpa Freeboota Orks who had been plaguing this part of the galaxy for years; finding their first serious fight (noted as a particularly great fight actually) but eating most of the Orks in the end, had two scout tendrils dodge around Trazyn's dynasty at Solemnance, and completely crushed the Death Shadows Chaos Space Marines at Sybari.

Upon learning this, the Imperium of Man suspended the Damocles Crusade that had been declared against the Tau Empire, both to reallocate resources and because the High Lords of Terra had decided that the Tau would serve as a nice buffer between them and this new threat. Marneus Calgar, the Chapter Master of the Ultramarines, took most of the Chapter to the Ultramar world of Prandium. However, the Tyranids were able to overwhelm the Ultramarines and force them to abandon Prandium. Calgar spent a week fasting and declared that the Ultramarines had suffered the sin of pride by allowing themselves to become complacent (bear in mind this was years before our spiritual liege was working at Games Workshop). Falling back to Macragge, the Ultramarines fortified their chapter homeworld, but the Tyranids responded by unleashing the Swarmlord. Despite the best efforts of his Honour Guard (including the force feeding sacrifice of their commander), Calgar was so grievously wounded that he lost both his arms and legs, one eye, and his stomach, and had to retreat to orbit, leaving the northern polar fortress with only the First Company to defend it. Fortunately, Imperial Navy reinforcements from Segmentum Tempestus (yes, the Tyranids were so bad they needed reinforcements from another Segmentum) finally arrived. The battle only turned in the Imperium's favor when the Emperor-class Battleship Dominus Astra detonated its warp-drive, dragging most of Hive Fleet Behemoth into the Warp and letting them meet the Chaos Gods*. Returning to Macragge, the Ultramarines Third Company scoured the northern fortress, where they discovered First Captain Saul Invictus and the entire First Company had been killed to a man. Only one member of the First Company, Battle-Brother Altarion, had survived the battle, and he was so messed up that he had to be put into a Dreadnought, which of course invites philosophical debate on whether he actually died or not, but that's for another day.

Aftermath

While the Imperium had won, the cost of Hive Fleet Behemoth was severe. An entire world of Ultramar was lost and the Ultramarines First Company with it. Captain Severus Agemann of the Second Company was promoted to First Captain of the Ultramarines, while Veteran Sergeant Cato Sicarius was promoted to Captain of the Second Company. The losses the Ultramarines suffered were so great that Dante sent several companies of the Blood Angels to help the Ultramarines purge the remaining Tyranids from Ultramar. Nearly a century later, the Ultramarines had still not fully recovered from the losses Hive Fleet Behemoth had inflicted, but by that point Macragge was fully free of Tyranids. Several ships of Behemoth survived, but without the coordination of the Hive Mind they broke apart into smaller Hive Fleets. The Swarmlord was officially listed on the rolls of the fallen, but its corpse was never located.

Despite all their losses the Ultramarines believed that they had crushed the Xenos threat and another enemy of the Emperor was no more. Oh how such hopes came to haunt them when two and a half centuries later, Hive Fleet Kraken assaulted the Imperium's borders...

[*] We can hope this doesn't come back to bite us later.

Hive Fleet Gorgon

Gorgon is one of the Behemoth splinter fleet had survived and nom'ed its way to the Eastern Fringe: The Tau Empire. Unlike other hive fleets, this one was relatively small, but was known for its fast adaption to new weapons and tactics to evolve the tyranid spawn. To fight against the Tau, knowing their cheesy plasma weaponry deals too much damage for their large biomorphs to handle, it decided to Zerg rush into the Fire Warriors firing line by shit out expendable Hormagaunts. Unfortunately, the Tau is also known for its battlefield adaption and evolving technology that they responded by switching to older or newer weapons when dealing with Gorgon's fast adaption. In the end Tau found that with high focus on fast adaptation Gorgon lacked more complicated Synaptic creatures (which required more time to modify), and declatred any-synaptic hunt that only Tau with their weaboo battlesuits, sneaky kroots and swarms of cheap aircraft could perform, so huge swarms of specialized gaunts Gorgon relied upon went feral and were easily dealt with. To add and insult to injury, Imperial Guard reinforcements lost in the Warp earlier duting Damocles Crusade arrived and allied with Tau to fight more eminent threat of Tyranids, further diversifying weaponry range to counter Gorgon adaptation.