Reaper's plague

hexproof.
At the beginning of each upkeep put a -1/-1 counter on each creature. When there are no creatures left in the battlefield, Sacrifice Reaper's plague.
''When he fled the battle, the dreaded plague reaper, left his unholy taint upon the fields, even the most skilled of healers couldn't purefy the toxic stain he had brought. damn that creature!'' - Khrishan, commander of the blue winged legion.