Thief of Sanity

Flying
Whenever Thief of Sanity deals combat damage
to a player, look at the top three card of the
player's library, exile one of them face down,
then put the rest into their graveyard.
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.