The Tyranids have come to the human galaxy in search of fresh genetic material to feed their Nom-Queen and revitalise the hive fleet after its long journey through space. However, not all captives are fed to the Nom-Queen; some are useful simply as food for the Tyranid bio-construcu. And some are sacrificed to the breeding programs of other creatures; it is their grisly fate to become hosts for immature grubs or the larvae of a bio-construct.
If the Tyranids want to store a captive so that he can be eaten, genetically shredded or used as a breeder host at a later date, then a special biocontruct called a Shroud-Spinner weaves an anaesthetising cocoon round the victim to keep him alive and fresh. This is the reason why traps assaulting a Tyranid ship sometimes find whole chambers of these cocooned victims.
One particular creature which feeds upon a captive during its larval stage is the parasitic Mind-Slaver. The mature MindSlaver is a crab-like bio-device which is used by the Tyranids to gain temporary control over another bio-machine which has malfunctioned or been damaged. As such, the Mind-Slaver takes over the individual mind of the bio-machine and allows the hive mind to control its mechanical functions.
The larval form of the creature is about the size of a pea or small pebble. When the egg laid by the Nom-Queen hatches, the creature is introduced to a living captive. It burrows into the captive's skull and searches out the brain stem. Here it clasps in place with its legs and attaches itself to the base of the medulla.
At first the immature creature is not powerful enough to influence its host but, as it feeds from the host's blood stream it becomes stronger, allowing the hive mind to seep into the host's brain and eventually take over and direct all of the creatures higher brain functions. These victims are known as mind slaves. Although they're captives, their actions and thoughts are completely controlled by the Tyranid hive mind.
Eventually the Mind-Slaver outgrows and destroys its host but, until it does so, the mind slave moves about the ship and performs tasks under the direction of the hive mind.
If the ship is attacked, the mind slaves will be amongst the first to move to its defence, especially if they belong to an armed warrior race whose fighting skills may be usefully employed by the Tyranids. As the Space Marines move through the spacecraft they may meet mind slaves controlled by the Tyranid hive mind and will recognise many of the creatures of the Warhammer 40,000 galaxy including Chaos Space Marines, Imperial Guard, Orks and Eldar.
The ultimate fate of a mind slave is to be sacrificed to the next part of the life cycle of a mind slaver. When it is quite large and almost fills the cranial cavity of its host, the Mind-Slaver exudes a chemical which encysts the host, dissolves him from the inside and ~ his body into nutritious soup. The MindSlaver feeds off this soup and grows into its adult form