Leyline of Transformation is a rare Enchantment card from Duskmourn: House of Horror (DSK), collector number 372. Current market price: $1.00 (foil $2.07), TCGplayer market prices via Scryfall.
If this card is in your opening hand, you may begin the game with it on the battlefield. As this enchantment enters, choose a creature type. Creatures you control are the chosen type in addition to their other types. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.