Mindless is a tag applied to some units, such as the and s. Mindless units are immune to mind-affecting spells like and . They also tend to have high Morale values that ensure that they will never rout: they will either win, or die trying. In situations where they would otherwise have to rout (such as not having any Commanders that can lead them), mindless units will immediately stop fighting and effectively die – this happens to your Blood Slaves, for example, when they'd otherwise remain usable by other Blood Mages if their initial master catches an arrow too hard.
This tag does not prevent units from casting spells, as demonstrated by the Pretenders. In fact, Mindless Mages are less vulnerable to having their spells interrupted by damage, since they have no train of thought that can be derailed.and several