Life thrives in the Hauntwoods, but it follows its own laws, unbound from the natural order.

Leyline of Mutation

If Leyline of Mutation 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.

Duskmourn: House of Horror

If Leyline of Mutation 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 that you cast.

Duskmourn: House of Horror Promos

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.

If a spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If you cast a spell for another cost "rather than pay" its mana cost, you can't choose to cast it for any other alternative cost. You can, however, pay additional costs. If the spell has any mandatory additional costs, such as that of Abhorrent Oculus, those must be paid to cast the spell.

If you cast a spell for which mana may be spent as though it were mana of any color, you may cast it for Leyline of Mutation's second ability's alternative cost and still spend mana as though it were mana of any color.