Azkaban, the Wizard Prison

When Azkaban enters the battlefield, exile
each Dark Wizard on the battlefield until
this leaves the battlefield. for each creature
exiled with card put a prison counter in this
At the beginning of your upkeep remove a
prison counter and create a 1/1 black
Dementor with flying and deathtouch,
if no counters were removed add a
prison counter to Azkaban