Kalran: Tales of the four nations/Undead infantry

Due to the undead heaving their souls forcefully tethered by the necromantic energy of the blight, and only controlled because of the elder necromancers in the Ziggurat, and not, well, living, the undead will not retreat when morale breaks, instead they will begin to lose health until morale is restored or they die. And if any undead units leaves the blighted area, and not under a necromantic aura, they will start taking unhealable morale damage, as the necromancers begin to lose their hold over their thrall, until they return to blighted land or necromantic aura, or dies, their souls finally breaking free from their physical prisons.