Thief of Sanity

Flying
Whenever Thief of Sanity deals combat damage
to a player, look at the top three cards of that
player's library, exile one of them face down,
then put the rest in their graveyard. For as long
as that card remains exiled, you may look at it,
you may cast it, and you may spend mana as
though it were mana of any type to cast that spell.