The Broodmind is the name given by a Genestealer Cult to the psychic gestalt collective consciousness shared by every member of the cult and directed by its Genestealer Patriarch. It is similar in function to the larger Tyranid species' collective Hive Mind, but unites all the human and hybrid members of a given Genestealer Cult.
The Patriarch unifies the will and purpose of its cult with a subtle alteration of the psyche through the implantation of Tyranid genetic material -- a subconscious link known as the Broodmind. It is this shared sentience that makes the cult so unusually tight-knit and loyal, that gives them uncanny strength and speed in battle, makes them so utterly self-sacrificing. Such traits, in the Imperium's view, undermine the spiritual sanctity of Mankind.
The Broodmind is initiated first in those members of the cult who receive the "Genestealer's Kiss," the introduction of Genestealer genetic material through infection by the Genestealer Patriarch's ovipositor. These people are fully human, but now come under the Patriarch's psychic influence. They are often referred to as "Brood Brothers."
The cult's hybrids, the offspring of those initially infected by the Patriarch who have been mutated from the human baseline, are born into the Broodmind. They are a part of its collective consciousness from the time of their birth.
The Broodmind is capable of altering the perceptions of those infected by it. For instance, the parents of each of the four generations of hybrids never see their children as monstrous, alien mutants. Even hybrid parents of the fourth generation since the infection, who can pass for fully human themselves, see their fifth generation offspring, Purestrain Genestealers, as adorable human infants.
The Broodmind prevents them from seeing these creatures as the bestial, hissing changelings they really are. These parents will defend their alien children with the fierce passion all mothers and fathers reserve for their offspring.
The Brood Mind also acts as a psychic beacon to draw a Tyranid 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, their hybrids and Brood Brothers 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: Genestealer Cults (8th Edition), pp. 7-8, 10, 113