In its larval stage the parasitic Mind-Slaver feeds upon the Tyranid's captives. It burrows into the victim's skull and searches for the brain stem. Here it clasps in place with its legs and attaches itself to the base of the brain.
At first the immature creature is not powerful enough to influence its host but, as it feeds from the blood stream, it becomes stronger, allowing it to take over and direct all of the creature's higher brain functions. These victims are known as mind slaves. Although they are captives, their actions and thoughts are completely controlled by the Tyranids.
If the ship is attacked, the mind slaves will be amongst the first to move to its defense, especially if they belong to an armed warrior race whose fighting skills may be usefully employed by the Tyranids. Invaders will find themselves being attacked by sentient beings controlled by the Hive Mind including Space Marines, Chaos Space Marines, Imperial Guard, Orks and Eldar.