Hive Fleet Leviathan
Leviathan is the third and largest Hive Fleet so far to have assaulted the galaxy in Warhammer 40k (you call tell by mentioning it as the "largest so far" GW is just planning to unveil a new bigger fourth one as a global summer campaign at some point), and the cause of the ongoing Third Tyrannic War. The hive fleet has struck below the galactic plane, rising upwards in a jaw-like formation swallowing whole entire sectors in the shadow of the warp. Holy Terra lays directly in their path and for a chilling reason; the Emperor, glowing as he does in the warp, is serving as a big beacon to the Tyranids which they can't miss, like a moth to a lightbulb. In fact, the canon pretty much says he is responsible for bringing the Tyranids to the galaxy in the first place, because he is acting as a draw to them. Way to go Emps.
Inquisitor Kryptman, the long standing authority on the Tyrannic menace, quickly acted when he suspected a new hive fleet was arriving and did several things that, while are extreme, make a hideous sort of sense. He ordered the exterminatus of countless worlds to deny the Tyranids vital biomass, really forestalling their advance. The exchange of this though was countless numbers of people have died and habitable worlds have been destroyed wholesale. Even the Imperium couldn't stomach this and the Inquisition has kicked Kryptman out. He was not done yet though. By capturing a brood of genestealers and sending them off into ork controlled space, he hoped the two species would destroy each other but it seems to be backfiring amazingly, as both sides seem to be growing constantly stronger through the conflict. The whole thing only ended when Ghazghkull dropped down into the sector and kicked ass.
Oh, and a shadowy bunch of Inquisitors called the Strategic Collective have given the High Lords a memo called the Stark Report which is causing them all to shit themselves because they are saying Leviathan is only a SCOUTING fleet and that the main fleet is still en route, but will most likely arrive in less then a century. They say if their findings are true, they will need to increase mobilisation of Imperial war elements by 500%… and that is being conservative.
Grim times indeed!
The Devastation of Baal
Following down the same road as its precursors, Hive Fleet Behemoth and Hive Fleet Kraken, Leviathan has now been largely destroyed, the Bio-Mass it absorbed and used in its creation lost to the Hive Mind. Although one Tendril remains mired in perpetual conflict with the Orks of the Octarius Empire, the largest Tendril has now been obliterated at Baal after the Great Rift opened up and dragged most of their ships into the Warp. At the same time, Ka'bandha and the army of Khorne Daemons he led showed up on Baal Prime to make it clear to the Hive Mind that he would not be robbed of his revenge.
Although the Hive Fleet devoured most of the Baal System, the Blood Angels were able to rebuild and resupply using the new Primaris Marines provided to them by the Indomitus Crusade, though it is said that they are still stretched thin. The Swarmlord died again in this battle, slain in single combat by an exhausted Dante.
Unfortunately, several of the Hive ships that the Great Rift swallowed up were expelled back into the Materium, allowing the countless new splinter fleets this created to extend as far as Segmentum Solar. Many of these splinters are said to possess entirely new biomorphs, possibly as a byproduct of Warp fuckery.
On top of that, the Tyranids have begun detaching elements of the Octarius tendril to form new tendrils, which have been greatly strengthened by their acquisition of Ork genetic material. It's gotten to the point where the Hive Fleet has occasionally left biomass behind for lesser Hive Fleets such as Kronos and Hydra to feed upon, as they are thought to have acquired all they need to begin their push to Segmentum Solar.
|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