In the later stages of the recent Shyedarian campaign of expansion and conquest, the Shyedarians developed several devices to ensure the obedience of their captives and conscripts. Easily the most widespread of these devices was the heart crystal: a small medical device in a crystalline shell implanted directly into the chest of the subject, leaving the glowing upper surface of the crystal visible as proof of installation. Not only were these devices implanted in prisoners, but also in nearly all Shyedarian military personnel as well, giving commanding officers an easy means of controlling their troops.
The function of the heart crystal was to control the flow of blood in the subject's body. Originally this was designed by Shyedarian medics to aid patients with poor blood circulation, but the technology was adapted by the military. Through the use of a remote control programmed with a particular heart crystal's access code, the controller could selectively stop or start the flow of blood in any part of the subject's body, causing pain, unconsciousness or even death. The use of these devices was widespread until the end of the Shyedarian Revolution; upon becoming part of the CIS, non-medical use of heart crystals was deemed excessively cruel, and was greatly restricted.
There were two versions of heart crystal. The basic heart crystal was simply implanted in the chest; it could be removed using basic surgical tools and skills, or in extreme circumstances simply by digging in and pulling it out (although this was highly painful and risky). The advanced heart crystal anchored itself in the subject's chest by extending microscopic tendrils that wove themselves throughout the tissues of the subject's circulatory system. This had the beneficial effect of strengthening those tissues, but it also made it impossible for the heart crystal to be removed involuntarily without causing massive, fatal damage. In order to be removed, an advanced heart crystal had to first be deactivated using proper codes, which would cause the tendrils to retract.