There is no greater way to convince your enemy that your army is invincible than tropps that refuse to stay dead.
When cast on a stack with dead members, Animate Dead creates a stack of the same type of creature on your side. Much like Create Illusion, this spell revives a certain number of hit points worth of creatures per casting, based on the power of your hero. Unlike with Illusions, the re-animated creatures possess all abilities and traits of the original creature, including spellcasting, and cannot be dispelled. As with the Raise line of spells, this can be cast on dead stacks, or on stacks with dead members, and the total that can be animated is limited by the number and hit points of the deceased creatures.
Animated creatures vanish after the battle ends, making them ideal for retaliation fodder and Sacrificial lambs.
Surprisingly, any creature (except those immune to magic) can be animated, even constructs, elementals, and the undead themselves.
This spell is probably best thought of as Create Illusion for Death. It has the usual downside (for Death) of requiring the targets to already be slain before you can copy them, making it more of a mid-battle spell, but the creatures that you get can use magic (if they were spellcasters), and don't have the vulnerability to dispelling that Illusions have.