Flamestrike

Flamestrike is one of the strongest attacks in the game, although it typically occurs lategame due to the economic investment (Mage's Guild and the associated upgrade). It is an AoE spell for the cost of 50 mana, doing 25 damage to the units in the line of fire. It takes 100 gold to learn in the Mages Guild, 200 gold if you are learning, or have already learned Heal. Be careful, though. Because it is possible to get harmed by your own Flamestrikes and you may accidentally end up offering up your mage for a Flamestrike blast. To avoid being decimated by Flamestrike, split your units up so that they're not all in the same place.