Shepherd of the Clouds

If the mana cost of a card in your graveyard includes {X}, X is 0 for the purpose of determining its mana value.

If you return an Aura to the battlefield this way, you choose what the Aura will enchant just before it enters the battlefield. An Aura returning to the battlefield this way doesn’t target the player or permanent it will enchant, so permanents or players with hexproof may be chosen; however, the chosen recipient must be able to legally be enchanted by the Aura, so a player or permanent with protection from one of the Aura’s qualities can’t be chosen this way. If there’s nothing legal for the Aura to enchant, it stays in the graveyard.