Melody: Mechanics
Delve:
You may exile any number of cards in your graveyard as you cast this spell. It costs 
less to cast for each card exiled this way.
Used on 14 cards:
Six Low Notes, Truffle Hog, Nakhtathan's Fury, Pinning Gale, Goyfgoyf, Release Tension, Dirty Deeds, Flickershift, Hollow Bones, Tombstalker, Transposition, Keeper of the Sacred Mind, Revoke Visa, Sacred Sifting
less to cast for each card exiled this way.
Used on 14 cards:
Six Low Notes, Truffle Hog, Nakhtathan's Fury, Pinning Gale, Goyfgoyf, Release Tension, Dirty Deeds, Flickershift, Hollow Bones, Tombstalker, Transposition, Keeper of the Sacred Mind, Revoke Visa, Sacred Sifting
Conceptually has little connection to the flavor of the set, but it makes things cheaper and uses the graveyard, which works with all the other mechanics.
Rhythm:
Whenever you cast a spell, you may untap this creature.
Used on 14 cards:
Circadian Protector, Drowsy Heron, Remora of Immigration, Sea Merchant, Ceremony Attagan, Sensorsilk Spider, Pulselight Wizard, Windshield Skyguard, Tremulous Aquamoeba, Waverider, Circadian Behemoth, Entrancing Echo, Refracting Pegasus, Karos, Regent of Themestria
Represents how creatures interact with music/magic, dancing and creating chained effects.
In card text or details pages, write the mechanic's code name (playtest name) between square brackets, like "[Crittercast]", "[Bushido 1]" or "[Delay 4 {2}{R}{R}]". It will be expanded to the mechanic's name plus reminder text, such as "Suspend 4 – 
. (Rather than cast this spell from your hand,...)"
To expand a mechanic's name but not its reminder text, include parentheses at the end of the square brackets: "[Crittercast()]", "[Delay 9 {R}()]".