Mind-Shriven are mortals or Daedra who have had their will and mentality stolen by Molag Bal, the Daedric Prince of Domination. They are not to be confused with Soul-Shriven, dead mortals whose souls have been stolen by Bal and replaced by a Daedric vestige. Unlike these "Vestiges", Mind-Shriven are not dead and have retained their mortal bodies.
Becoming a Mind-Shriven involves drinking a poison known as the Blood of Coldharbour, which will slowly begin to rob the imbiber of their willpower. This process can be delayed through alchemy, but not halted. Mind-Shriven can be identified by the wispy blue veins of cold fire that emanate from their eyes and veins. Once the transformation is complete, the victim takes on a lifeless, pallid tone of blue and becomes mindlessly aggressive. The process is not limited to the mortal races, as even horses and wolves have been known to become Mind-Shriven. Even lesser Daedra can become Mind-Shriven, as in the case of Mind-Shriven Scamps, unusual specimens which are often found in the company of other Mind-Shriven creatures.
The use of Mind-Shriven was most prominent during the Daedric occupation of the Imperial City in 2E 582 as part of the Planemeld. The Imperial Prison was used by the Daedra as a laboratory for experimenting with the process of stealing a mortal's psyche, and members of the traitorous Legion Zero were required to drink the Blood of Coldharbour to prove their loyalty to Molag Bal. Many legionnaires opted for death instead.
For game-specific Mind-Shriven information, see ESO.