Cranial Leeches are alien, grub-like creatures that have been linked to many mysterious deaths and heretical actions across the Askellon Sector. They are suspected as the cause when individuals have acted in ways antithetical to their prior behaviour patterns or in seemingly irrational ways.
Sector legend holds that some grotesque xenos species grows the leeches, which detach and seek out sentient life as their prey. Others hold that the leeches are bioengineered weapons created by some unknown alien people that either departed Askellon long ago, or uses the leeches as part of their advance invasion forces.
Most tales relate that once a leech implants itself inside cranial tissue, either slowly insinuating itself under skin or forcibly burrowing though the eyes or other soft entry points, its host falls under alien domination or is easily manipulated by any around him.
Some accounts have the leeches originating along Thule's trailing edges, from one of the many worlds still unexplored in Wilderness Space there. Many believe, though, that they are grown in vats and sold through the Faceless Trade, and several Inquisitorial actions have been made to find and destroy these sources or gain control of them for use in interrogations.
While some raids have been successful, the continued appearance of the leeches across the sector indicates other sources still remain.
A leech can be applied to a person's skin, and requires roughly five solar minutes to make its way into a brain and spread its tendrils across neural tissue, producing agonising pain in the host as it does so. The host becomes highly suggestible, and carries out any instructions without hesitation unless they are immensely strong of will.
To an onlooker, a victim might also begin acting oddly, performing actions that could represent the instructions of his alien masters.
Removing an implanted leech without damaging the host requires intense medical skill and ability, as the leech causes irreparable trauma to the brain at the slightest failure. Otherwise, if there is no concern for the host's well-being, a leech can be forcibly and easily removed.
Removed leeches die within solar seconds, no matter the means used.
- Dark Heresy (2nd Edition): Enemies Without (RPG), pg. 59