The Throne of Winter

Play with the top card of your library revealed.
You may cast the top card of your library if it's
an instant or sorcery card.
Instant and sorcery spells you cast cost {u} less to
cast. This effect reduces only the amount of
colored mana you pay. (For example, if you
cast an instant spell with mana cost {2}{u}, it costs {2} to cast.)