A Hive Fleet is a swarm of Tyranid bio-ships that travel together. They roam through space, invading planets and consuming their biomass to reproduce.
Nobody knows where the Tyranids come from, but it's definitely outside of the galaxy. The first major waves of Tyranids came from the galactic east (Ultramar, the Tau Empire, etc.), but the latest invasions occurred in the galactic south. At first, they seemed to be occurring at random, as systems would just fall silent without anything happening to their neighbors, but then it became clear that Hive Fleet Leviathan was attacking from below the galactic plane. Think of Leviathan as a really big, space Jaws, attacking and eating planets from below.
Most of the mass of a Hive Fleet is in its Hive Ships, massive biological factories that take in biomass from conquered planets and turn it into more Tyranids. Each one contains one or more Norn Queens, who sift through the biomass looking for interesting traits and genes that they can graft into their "children", chosen based on the obstacles they faced. Over time, this directed mutation and evolution results in different hive fleets having very divergent variants of warrior organisms and distinct styles of invasion.
The Hive Ships are protected by various strains of cruisers and droneships. Razorfiend Krakens hunt enemy ships, while escort drones cluster around hive ships as a living protective screen.
The Hive Fleets are rounded out by a pair of ship classes that they use to identify targets and get there in a reasonable amount of time. They send Vanguard drones ahead of the fleet, loaded with Genestealers, Lictors, and other infiltrator organisms, to candidate systems. Infiltrators that find their way to planets with lots of biomass found cults or colonies whose psychic networks act as beacons in the Warp and attract the attention of the fleet that spawned them.
Once a destination is determined, the fleet has to get there before it starves to death (analagous to the fuel limitations of other fleets, though unimpressive-sounding at best). It does this by employing a ship called a "narvhal" which somehow harnesses the target planet's gravity to bend space-time into a "corridor", which allow the fleet to travel to this planet at faster-than-light speeds (not as fast as a Warp-capable ship, but certainly faster than drifting at sublight). Presumably, this employs some sort of incredibly sweet facial horn. This bending has the fortunate (?) side-effect of causing tectonic upheaval on the target planet, disrupting any defenses but also warning anyone who knows what they're looking for that the fleet is coming for them. Narvhals have delicate gravimetric senses which are easily overloaded by nearby planets and stars, so the corridor dissolves as the fleet approaches its destination, and the fleet makes its final approach at sublight speeds, spending years or even decades in real-space. The fleet may hibernate to cool down and evade detection, only waking up when they are too close to be stopped.
Famous Hive Fleets
Games Workshop used to publish experimental rules for Genestealer cults and deep striking "Seedling Swarms", but from Fourth Edition onwards, Tyranids have been a strictly one-codex army. The only reason to choose one Hive Fleet over another is for the color scheme. Genestealer cults now have their own codex, but they aren't considered battle brothers to regular 'nids. Which makes sense, considering they end up getting eaten anyway.
8th edition changed this with the introduction of Hive Fleet Adaptations, the Tyranid version of Chapter Tactics and they're one of the best internally balanced yet.
- Lexicanum's list of all canon Hive Fleets
- Tyranid Hive Fleet Creation Tables, a set of roll-tables for generating your own Hive Fleet
|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