Box of Plethora

Box of Plethora enters the battlefield with an
amount of Plethora counters on it equal to the
highest loyalty of planeswalkers on the battlefield.
Whenever an opponent attacks with a creature
with lower power than the amount of Plethora
counters, the attacking creature becomes exiled
into the Box of Plethora at the end of combat.
When Box of Plethora leaves the battlefield,
return all exiled creatures to their owner's hands.