Voidstorm

When Voidstorm enters the battlefield, exile the
top X cards of your library face down.
Whenever a player plays a creature or artifact
spell, reveal a facedown card exiled by Voidstorm
at random. If the revealed card's mana cost is less
than that spell's, counter it. The controller of
that spell may play the revealed card for free
until end of turn.
{1}{u}{u}, Exile Voidstorm: Counter target spell.