Tyranids (often abbreviated Nids) are eusocial aliens invading the Milky Way in the Warhammer 40,000 setting. Unlike most other Warhammer 40,000 factions, the Tyrands aren't a futuristic version of a faction in Warhammer Fantasy Battle. They can trace their origins back to the Arachnids of Starship Troopers.
Tyranids are a group of organisms that come in a wide variety of forms, from worm-like scavengers to multi-kilometer-long creatures capable of interstellar travel. Many ground combat vectors of Tyranids are bilaterally symmetrical with six appendages, but there is a lot of variety among the many Tyranid strains.
Most Tyranids are not sapient and don't make use of technology, although they have at their disposal various organs and organisms to serve as weaponry. Combat vectors generally have a carapace comparable in durability to armor typically found in the Warhammer 40,000 universe.
The typical behavior of Tyranids is for large groups of space-based organisms (known has Hive Fleets) to move from star system to star system in search of inhabited planets. Upon discovery of suitable planets, the Hive Fleet will deploy large numbers of fighting creatures to destroy any threats while other organisms devour all of the planet's useful resources (consisting primarily of the planet's biosphere, but including the atmosphere and crust minerals). The fleet then reabsorbs its creatures and all of the material they have consumed through links known as capillary towers. When the the operation is complete, the hive fleet moves on in search of other worlds on which to feed, leaving the formerly habitable planet barren and lifeless.
- Hive Tyrants: Primary "synapse" creatures (command and control)
- Tyranid warriors-Elite soldiers and secondary "synapse" creatures
- Gaunts : Standard Tyranid infantry with many variants (Termagaunts, Hormagaunts, etc.)
- Genestealers: Elite infantry and biological infiltrators
- Carnifex: Heavy assault
- Zoanthropes: Psionic artillery
The true origin of the Tyranids is unknown; they are invaders from outside the Milky Way galaxy. They might have somehow evolved naturally, been created like the Orks as biological weapons gone wrong, or actually be the agents of some other alien power.
The first major incursion of Tyranids was Hive Fleet Behemoth. Behemoth hit hard, devouring systems one at a time, including a few Imperial Worlds, before entering the territory of the Ultramarine Chapter of Space Marines, known as Ultramar. The fleet eventually decended upon the Capital of Macragge, where the Ultramarines managed to destroy the hive fleet, albeit with a massive losses of personnel and equipment.
The second major incursion of Tyranids was Hive Fleet Kraken. Unlike Behemoth, Kraken broke up into numerous smaller fleets to attack multiple worlds at once, augmenting it's forces with Genestealer cults, allowing it to more quickly devastate large swaths of Imperial space, although the smaller forces were easier to resist. The ravaging stopped when the majority of Kraken's forces descended upon the Eldar Craftworld of Iyanden: much of the Craftworld was devastated, and millions of Eldar were killed, but the attacking Tyranids were eventually repulsed. Most of the shattered remains of Hive Fleet Kraken are currently moving upon the fringes of the Tau Empire.
The most recent Tyranid incursion is Hive Fleet Leviathan, which is far larger than the previous two hive fleets. Leviathan approached from "beneath" the galactic disc, rather than from the edge, striking deep in the heart of the Imperium without warning. Much of the fleet is currently moving towards Holy Terra. Whether this is the entirety of the Tyranid forces or there are other (and possibly larger) Tyranid fleets outside the Milky Way is unknown.
All Tyranids possess some level of psionic ability and follow the telepathic instructions of a collective consciousness known as the "Hive Mind". Particularly advanced Tyranid forms --known as "synapse creatures" --serve as command hubs for less intelligent Tyranid variants. Destruction of synapse creatures in an area can cause lesser strains to lose coordination, go insane, and even turn on each other and rip themselves to pieces.
Tyranids regard all other forms of organic life as sources of food and genetic material that they can potentially incorporate into their gene pool. Various Tyranid weapon creatures can expel projectiles or energy bursts as effectively as an advanced assault rifle or laser weapon. These weapons scale up much as technological weapons do, ranging in size and effect from small arms to ship-to-ship combat weapons. Tyranid synapse creatures and lesser Tyranids under their influence are completely fearless, fighting to the death on behalf of the Tyranid hive mind. While individual Tyranid creatures may be unintelligent, the hive mind is highly intelligent, directing its resources to execute a well-planned strategy. Tyranid hive fleets can consist of hundreds of space-faring life forms and billions of smaller Tyranid forms.
Tyranids bear a resemblance to the Zerg from Starcraft, although they technically predate that game. The first publication of Warhammer 40,000: Rogue Trader included Tyranids, although the nature of the army has changed considerably over time.