Morven, Shoreline Scourge

Swampwalk (This creature can't be blocked
as long as defending player controls a Swamp.)
Whenever Morven deals combat damage to a
player, look at the top four cards of that player's
library, exile one of them face-down, then put
the rest on the bottom of that library in a random
order. You may look at and cast that card for as
long as it remains exiled, and you may spend
mana as though it were mana of any type to cast
that spell.