Legendary Creature — Kirin Spirit
Flying Whenever you cast a Spirit or Arcane spell, destroy all permanents with that spell's converted mana cost.
If the Spirit or Arcane spell has in the mana cost, then you use the value of on the stack. For example, Shining Shoal costs . If you choose X = 2, then Shining Shoal’s converted mana cost is 4. Shining Shoal also has an ability that says “You may exile a white card with converted mana cost X from your hand rather than pay Shining Shoal’s mana cost”; if you choose to pay the alternative cost and exile a card with converted mana cost 2, then X is 2 while Shining Shoal is on the stack and its converted mana cost is 4.