Some spells, such as Queen Medusa (Snake Magic), give you a chance to Petrify a monster. 

When a monster is Petrified, it turns to stone. A countdown appears above its head showing the remaining duration. While Petrified, it takes increased damage.

Tip: Smaller monsters, such as the Evil Rat or Frog, are more susceptible to being Petrified.

Learn more about: Challenge - Defeat X Petrified monsters