Vorkuhn, Architect of Decay

Whenever Vorkuhn enters the battlefield or
attacks, put a rot counter on each other nonland
permanent. Then, destroy each permanent with
mana value less than or equal to the number of
rot counters on it.
Whenever a nonland permanent an opponent
controls is put into the graveyard from the
battlefield, you may return a nonland permanent
with lesser mana value from your graveyard to
the battlefield.