Jace's Sanctum

The reduction is the part everyone notices; the scry is the part that built the deck. Instant and sorcery cost-reducers are an old idea, and most of them stop at the discount: shave a mana off each spell, let the velocity speak for itself. The second clause is what turns this from a tempo enabler into an engine. Every spell you cast filters your draws, so a deck full of cantrips and burn spends the game looking deeper into itself, paying less each turn while smoothing the top of the library. That feedback loop is the whole point: the more spells you have to fuel, the more the discount and the dig compound, which is why this card lives in spell-dense shells rather than as a generic value piece. It asks nothing of your creatures and rewards a build that treats the battlefield as a side effect. The catch is that it sits idle until you follow it with a spell, and it folds to enchantment removal, so it is a payoff that demands you have already committed to a deck whose plan is "cast a lot of spells, win on accumulated card quality." Get there and it does both halves of the job at once: it makes the deck cheaper to run and steers it toward its best cards, which is a more dangerous combination than a flat cost reduction ever was.