Hive Fleet Tiamet
Discovery of Tiamet
Hive Fleet Tiamet was first encountered around the Tiamet system, on the eastern side of Ultima Segmentum. Considering this region is quite unremarkable for any of the major powers of the galaxy beforehand, the Imperium made first contact with it through a small Explorator Fleet, technically making this the Imperium's first contact with a Tyranid Hive Fleet. They didn't consider the presence of the tyranids in the worlds around Tiamet something threatening, believing they were just the natural fauna of a Death World. Because the Imperium had other things to worry about, and because the area had no real value, it was left undisturbed (they did throw a symbolic bombardment of the place once, though, to no real effect).
It wouldn't be until a fleet of Eldar explorers when someone recognized the Tyranids for what they were. However, they never reported it: the Shadow in the Warp had grown so powerful in the region that they became comatose.
Behaviour of Tiamet
Regular Tyranids follow a nomadic lifestyle, travelling though the galaxy and devouring whatever planet or biomass source they can find, usually following the psychic beacons activated by the multiple genestealer cults spread throughout the galaxy, leaving wherever they go bereft of life. Hive Fleet Tiamet follows a radically different lifestyle. For some reason, these Tyranids permanently inhabit the Tiamet system and its surrounding planets and don't fully strip them of biomass. And they guard the place very zealously, too. Every time a merchant or an exploration fleet gets close to the system, it gets immediately swarmed by the nids, who drag it off to their main stronghold on the world of Ziaphora. The genestealer cults serving them act similarly, with their leaders instructing their cults to hijack nearby ships so they can travel to a "paradise planet" within the system.
Eventually, so many ships were lost in the region that a Kill Team was sent to explore the area to figure out what had been going on over there.
When the Kill Team arrived to the place, they discovered a massive organic towering structure (as in, the size of a continent) on Ziaphoria. Nobody's sure what the hell it's supposed to do, but it's psychic and generating such a powerful Shadow in the Warp that the Kill-Team's Librarian actually had his head explode from the strain. The resulting screams attracted a massive swarm of Tyranids to the Kill-Team's presence, and its leader just barely had enough time to warn their Corvus Blackstar about what they discovered before being torn to bloody ribbons.
Whatever this thing is, everyone agrees that if the flesh-tower is important enough for an entire Hive Fleet to be assigned to guard it, then it can't be good for the rest of the galaxy. The current speculation is that it's a beacon meant to attract more Hive Fleets, which could easily spell disaster should it be finished. And all the while, Tiamet keeps sending out tendrils to nearby worlds, gathering more biomass to complete its creation...
The Deathwatch is extremely interested in taking this thing down, and has contacted excommunicated Inquisitor Kryptman to find out what it is and how to get rid of it. This will undoubtedly be a difficult task- Hive Fleet Tiamet has evolved to excel at defensive measures, and its bioforms possess a heavily reinforced exoskeleton that lets them shrug off volumes of fire that would kill any other Tyranid creature twice over. The solution is probably going to be Cyclonic Torpedo spam...assuming the Hive Ships don't intercept the exterminatus fleets first.
|Tyranid Hive Fleets|
|Major Hive Fleets:||Hive Fleet Behemoth - Hive Fleet Kraken - Hive Fleet Leviathan|
|Minor Hive Fleets:|| Hive Fleet Colossus - Hive Fleet Dagon - Hive Fleet Gorgon - Hive Fleet Hydra - Hive Fleet Jormungandr |
Hive Fleet Kronos - Hive Fleet Moloch - Hive Fleet Naga - Hive Fleet Ouroboris - Hive Fleet Tiamet