Prove Your Worth

When Prove Your Worth enters the battlefield,
put three timer counters on it. At the beginning
of your upkeep remove a timer counter from
Prove Your Worth.
Whenever a creature destroys another creature,
put a proof counter on it.
When the final timer counter is removed from
Prove Your Worth destroy each creature with no
proof counters then each creature loses its proof counters. Exile Prove Your Worth.