Perhaps there should be a mortal incarnation of this Dark One... a necromancer or lich king of some sort that the Lightbringers could expressly find and target, instead of accusing each other of being enthralled? You'd need to keep him or her hidden for that reason. Actually, being a lich king/queen could be especially interesting, in that you'd need to make and hide phylacteries that the lightbringers would want to hunt down.
Or, if you want to keep the lose condition simply being a turn-counter, you could make it so that raising undead armies requires a focus of some sort that the lightbringers could find and destroy to destroy your whole army at once. That'd be a major setback, after all, making the timer a threat.