Some spells, such as Voodoo Queen (Water Magic), give you a chance to Curse a monster. 
When a monster is Cursed, it turns purple. A Voodoo Doll icon and turn counter appear in front of it, showing how many cards you can draw before the effect ends. 
When a monster is Cursed, Voodoo Dolls may appear on certain cards. Notice how the 8 card changes from a normal card to one carrying a Voodoo Doll while the Curse effect is becoming effective.
While the Curse is active, every Voodoo Doll card you play deals bonus damage to that monster.
Other spells that may also give a chance to stun a monster include:
- Water Book (Water Magic)
Tips: Water Magic is most effective on Fire Mountain. Smaller monsters like the Pufferfish or Water Jelly are more susceptible to being cursed. 
Learn more about: Challenge - Defeat X Cursed Monsters