Nature's Rhythm is a rare Sorcery card from Tarkir: Dragonstorm Promos (PTDM), collector number 150p. Current market price: $3.31 (foil $6.82), TCGplayer market prices via Scryfall.
Search your library for a creature card with mana value X or less, put it onto the battlefield, then shuffle. Harmonize {X}{G}{G}{G}{G} (You may cast this card from your graveyard for its harmonize cost. You may tap a creature you control to reduce that cost by an amount of generic mana equal to its power. Then exile this spell.)