Justice is a fan-made global mod.

The undead aren't particularly aligned with the Ashan faction. They are dead and long cursed. I don't remember any instances of undead being holy.

Well, Ashan is the Goddess of Life and Death, who also controls time (there was something like that), and the undead are closer to her than anyone else. The problem is that the undead are animated by necromancy and dark magic, not by resurrection or Light magic. Because of this, they are vulnerable to Light but not vulnerable to Darkness. Ideally, it would be logical to make the red tier of the Empire immune to the "Unholy" spell, because they are kind of enhanced by demons (Biara was in power, demons are everywhere, and the crazy Alaric is fleeing into the sunset), but that would be imbalanced. The Orcs have a similar problem: the mages mixed humans and demons, and as a result, they have resistance to magic but vulnerability to both "Holy" and "Unholy" spells. It would be good to give them 50% resistance to one and 50% to the other, although the opponent probably won't use both spells in the same battle. There aren't many demon-worshipping mages; the well-known ones are Menelag and Fenrig. Arantir killed the rest, so it makes sense to leave them alone