Leyline of Mutation is a rare Enchantment card from Duskmourn: House of Horror Promos (PDSK), collector number 188p. Current market price: $0.45 (foil $0.40), TCGplayer market prices via Scryfall.
If this card is in your opening hand, you may begin the game with it on the battlefield. You may pay {W}{U}{B}{R}{G} rather than pay the mana cost for spells you cast.