Maftirri, Master Thief

Whenever Maftirri, Master Thief deals combat
damage to a player, look at the top three cards of
their library. Exile one of those cards face down
and put the rest into their owner's graveyard.
You may cast that card for as long as it remains
exiled, and you may spend mana as though it
were mana of any color to cast that spell.
"What trinkets have you brought me today, my dear?"
—Crime Lord Gonti