Some spells, such as the Snake Bite (Snake Magic), give you a chance of Poisoning a monster. 


When a monster is Poisoned, it turns green. A Venom Vial icon and a turn counter appear in front of it, showing how many cards you can draw before the effect ends. While Poisoned, each card you draw deals extra damage to the monster.


To complete the challenge that asks you to "Defeat X Poisoned monsters", you must kill a monster WHILE it is still Poisoned. 

Notes: If the monster is no longer Poisoned when you kill it, then it WON'T count for the challenge. 

Learn more about: How do I Poison a monster?