Xurveth, Plodding Waste

When Xurveth enters the battlefield, exile X
non-land permanents with prison counters on
them.
You may put a card with a prison counter from
exile into it's owners graveyard. If you do, add {2}.
Xurveth's power and toughness are equal to 1 +
the number of cards in exile with prison counters
on them.