Ebondeath Plague

Enchant creature you control
Enchanted creature has Poisonous 1
Whenever a creature block of become blocked by
enchanted creature, if it doesn't have an aura
named Ebondeath Plague attached to it, its
controller create a copy of Ebondeath Plague and
attach it to that creature."
At the begining of your upkeep, put a -1/-1
counter on enchanted creature.
When enchanted creature dies, you get a poison
counter.