Few would believe that of all the enemies faced by Mankind, a greyish vine-like plant could pose such an insidious threat. Variously referred to as "slave vine" or "the grey weed," but most commonly the Brain Leaf, this malignant creature was once thought to originate from the jungle Death World of Catachan.
In physical form, the Brain Leaf plant resembles a coarse vine or ivy, each tendril of which is tipped with a single fleshy leaf. Uniquely, the Brain Leaf possesses a form of gestalt consciousness, and each leaf is a macro-cell forming part of its utterly alien intelligence. The underside of each vine leaf is barbed to latch onto creatures straying too close and, once attached to a victim, microscopic fibre bundles penetrate the skin and extrude fibrous growths throughout a victim's nervous system to take control of its body utterly.
Though not sentient in any measurable sense, the Brain Leaf efficiently and swiftly dominates its victim, subverting it to the will of the parent plant. Once this has been accomplished, the leaf detaches from the main body of the plant and remains as an attached parasite on the victim, allowing a new leaf to rapidly grow in its place.
The victim, now a staggering thrall to the Brain Leaf plant, is used by the weed to protect itself and proliferate its young. Infestations of Brain Leaf have been discovered on many worlds, and recent emergent evidence is that the Brain Leaf is no true native of Catachan, but likely a rogue mutation and evolutionary descendant of the Tyranid Cortex Leech.
This makes the destruction of any infestation a priority for many in the Ordo Xenos of the Calixis Sector. The horrors visited on members of the sector's nobility during the infamous "Slaughter Gardens of Quaddis" incident in the late 41st Millennium have emphasized the need to prevent the spawning and multiplication of this foul vine.
- Dark Heresy: Core Rulebook (RPG), pg. 346