The Hive Mind is the gestalt collective consciousness of the Tyranid species. It is a nearly omniscient entity composed of pure psychic energy that originated outside of the Milky Way Galaxy. The Hive Tyrant is said to be a living vessel for it. It controls every Tyranid creature in a mental vice-like grip and directs their every action.
This control can be disrupted, however, with the death of the Hive Mind's synapse creatures through which it transmits and augments its psychic commands, such as when a Hive Tyrant's death leaves the lesser Tyranid bioforms to act completely on basic animal instinct.
On the battlefield, certain large and relatively autonomous Tyranid creatures, such as the Hive Tyrant or the Tyranid Warrior, are able to telepathically exert the Hive Mind's influence in order to control smaller Tyranid bioforms, which without the Hive Mind's psychic direction would revert to the instinctive, non-sentient behaviour of feeding and self-preservation. These synapse creatures focus the Hive Mind's psychic gestalt into nodal points that quell the natural instincts of nearby small Tyranid bioforms and are an essential component for a Tyranid swarm's effective command and control.
The Hive Mind is so staggeringly complex in its power and construction that it either drives most psykers insane or kills them. The only known agent of the Imperium of Man believed to have looked upon the Hive Mind and retained his sanity is Chief Librarian Tigurius of the Ultramarines. This could be due to his uncanny talent for making accurate predictions in general, but the accuracy and frequency of his knowledge concerning the intentions of the Great Devourer seem to show he has indeed tapped into the Hive Mind, a feat thought possible only by a psyker possessed of power equivalent to that of the Emperor of Mankind.
The Hive Mind also has the ability to block interstellar communications and travel through the Warp due to the potent psychic presence it creates in the Immaterium, essentially disrupting the normal flow of psychic energy within the Immaterium in much the same way as a Warp Storm. This ability has become known as the "Shadow in the Warp"; the Hive Mind uses this power when it unleashes a Hive Fleet, cutting off astropathic communications to the besieged part of the galaxy and forcing vessels already in the Warp in the vicinity of the Hive Fleet to drop back into realspace, thus making a call for reinforcements impossible.
This invasive psychic shroud sows terror and confusion on prey worlds as it traps and prevents the doomed inhabitants from calling for assistance. It even penetrates into the minds of nearby creatures and manifests as a primal malaise that can never be expelled. If a creature possesses psychic potential, then the horror they feel is magnified tenfold. Any use of their psychic abilities could unleash a form of incurable psychosis. As a Hive Fleet approaches a target planet, whole populations plunge into madness and despair even before the first Tyranids have made planetfall.
A smaller and more localised version of the Tyranid Hive Mind is created by the interlinking telepathy of a Genestealer brood. This so-called "Brood Mind" is used to control the host victims impregnated with Genestealer genetic material and create a Genestealer Cult.
The Brood Mind also acts as a psychic beacon to draw a Hive Fleet towards the planet where its cult is located. Once the summoned Hive Fleet comes within range of a few hundred light years of the infected world, a subliminal reaction is triggered in the Brood Mind to goad the Genestealers into a frenzy of killing. This unleashes a violent uprising, which weakens the defences of the targeted world and makes it easy prey for the arriving Hive Fleet of the Great Devourer.
- Codex: Tyranids (5th Edition), pp. 6-7
- Space Hulk Mission Book (3rd Edition), pp. 46-47
- Battlefleet Gothic Armada, pg. 80