Hymn of the Wilds is a mythic Conspiracy card from Conspiracy: Take the Crown (CN2), collector number 7. Current market price: $0.26 (foil $2.91), TCGplayer market prices via Scryfall.
(Start the game with this conspiracy face up in the command zone.) The first creature spell you cast each turn costs {1} less to cast. You can't cast instant or sorcery spells.