Necromancer – “Invasion” [Campaign 3, Mission 3]

If you use Power Mana on a unit that isn't a Necromancer, but on a unit of undead, the undead won't disappear. The question is, it's usually easy to target a Necromancer with Power Mana, as he has a lot of spells and, ultimately, only he has Necromancy, which will provide additional undead reinforcements.