I agree, I was also scared at first when I saw the wheel, but then I realized that pure mages are not lucky in the world of Ashan. The most optimal option is a ranger, with enchanted arrows, ballistas, pyrokinesis, etc., and the strongest mage is a barbarian, because his shouts cannot be canceled by anything, and his resistances, even just one skill, "Strength against Magic," destroys the entire strategy of playing as a mage.

Well, here it simply means that if a necromancer hero commands living troops, it is quite possible that Arantir will fight with Heaven's troops against Biara. In general, this skill is given to diversify tactics, but I can't imagine where a necromancer would be found who would change his standard troops for living ones.