The Imperial sigil used to designate a Tyranid Hive Fleet
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.
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 to have looked upon the Hive Mind and retained his sanity is Chief Librarian Tigurius of the Ultramarines. 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 that has become known as the "Shadow in the Warp"; it uses this ability when it unleashes a Hive Fleet, cutting off astropathic communications to the besieged part of the galaxy and thus making a call for reinforcements impossible.
Sources
- Codex: Tyranids (5th Edition)