The mutant becomes a total automata, obeying every command of whoever holds sway over it. (Its Int becomes zero.)
It can no longer use weapons of any type and any spellcasting abilities are lost (even those gained as a result of other rewards or attributes). Furthermore, it can no longer act as a unit or warband leader - command passes to the next most able individual. The mutant becomes immune to all psychological effects, no matter what their cause.
A Champion Of Chaos who becomes Mindless automatically turns into a Chaos Spawn.