Arlen, Cutthroat Thief

{1}{u/b}: Arlen, Cutthroat Thief can't be blocked
this turn.
Whenever Arlen, Cutthroat Thief deals combat
damage to a player, look at the top five cards of
that 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.