Grolnok, The Omnivore

Whenever a Frog you control attacks, mill three
cards. Whenever a permanent card is put into
your graveyard from your library, exile it with a
croak counter on it.
You may play lands and cast spells from among
cards you own in exile with croak counters on
them.
Grolnok's sonorous warcry terrifies all who hear it.