Ambitious Lieutenant

Whenever a creature you control deals combat
damage to an opponent, exile the top card of
that player's library. If the exiled card has a
mana value of 3 or less, you may cast it as long
as it remains exiled and you may spend mana as
if it were mana of any color. Otherwise, its owner
may put the exiled card into their hand.
"Sooner or later, my dear, you're going to bite off more
than you can chew."
-Baiere, Dark Librarian