Doomspeaker Clairvoyant

Whenever you expend X, where X is the number
of cards in your hand, exile the top card of your library. Until end of turn, you may play that card.
(A player expends X as they spend X total mana to cast spells during a turn.)

Whenever an opponent expends X, where X is
the number of cards in their hand, Doomspeaker Clairvoyant deals X damage to that player.