Shadow Sickness is a mental disease caused by either intense exposure to Shadowfell energies, or prolonged exposure to less intense energies.
About
Shadow Sickness that is inflicted by a powerful shadow creature also allows that creature to dominate and charm the target, taking control of their destroyed will. However, this side-effect of charm CAN be over-ridden by using other charming effects and powers, though it does not restore the creature to full health, just simply gives the "puppet strings" to another wielder.