Strix Serenade Not much to say here
If you can cast your spells more reliably than your opponents, your cards become more effective and you capture the equity of your opponents stumbling or taking too long to set up their plays
They are calling this the rarity key, and the rare, super rare and legendary symbols will be holographic on the cards (or some kind of shiny gradient), to make it even easier to differentiate
its emblem is one of the best card draw payoffs possible
I tried out different numbers and amounts of life loss, but the Sleeper Agent problem was quite real here: it was hard to get this at a rate where it was fun while encouraging you to keep it or give it away in different games
Soulherder shoehorns you into offering some support for Blink decks, but the options here are slim and most Cubes are going to feature creatures worth blinking anyway