Sworn enemy.

A sworn enemy is a great thing. At first, I didn't pay much attention to it. But in one final battle, I let the computer control the battle through the "autobattle" option, and when I saw how the dragons flew up to the enemy, breathed once, and took down 3 or 4 enemy stacks in one go, I couldn't believe it. Then I read about the sworn enemy's abilities in the descriptions, and it's true, it can take out several units standing next to each other at once.