But what if the player has warriors with no magic resistance? It's a spell for extreme situations; if there is resistance, use it on health, but otherwise, it's the same as playing as demons and casting a holy word against necromancers.
It's suitable for extreme situations. If Armageddon completely clears the battlefield, the winner is the one who cast Armageddon. He will be able to buy back his hero in the tavern.