The Brainleaf[1a] (also referred to as the "Grey Vine" or "Slave Vine") is native to Catachan and appears to be a small tree. It has a rudimentary animal brain with the lowest level of intelligence and appears to be able to control animals and humans through attaching its leaves to them. The leaves can float away from the tree for many metres using a wing-like undulation, but when they come into contact with a creature, the nerve bundles in the leaf attach themselves and grow throughout the victim's nervous system, effectively taking them over. The Brainleaf can then make use of the creature and its abilities.[1a]
There is another variant of Brainleaf that appears as a grey, hairy vine wrapped around a tall object. When prey gets too close to the vine, it will reach down and attach a leaf to the back of the creature's neck. The fibers in the leaf then quickly burrow into the spinal chord of the victim, reducing a victim to a brainleaf thrall. 
The Brainleaf uses its victims to protect itself, and propagate its young. Victims have no intelligence, often lacking the motor skills to use primitive weapons. To protect the Brainleaf, the Brainleaf Thrall will charge at anything that gets too close and beat it with whatever appendages it has.
The Brainleaf can be removed from a victim, though it is incredibly dangerous to the victim. If they aren't killed by the agonizing procedure, they will suffer permanent nerve damage. 
- 1: Warhammer 40,000: Rogue Trader
- 2: Dark Heresy Core Rulebook, pgs. 346, 347