Celestial Kirin
Legendary Creature — Kirin Spirit
Oracle Text
Flying Whenever you cast a Spirit or Arcane spell, destroy all permanents with that spell's mana value.Rules for this card
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A keyword ability that restricts how a creature may be blocked. See rule 702.9, “Flying.”
Rulings (1)
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Wizards of the CoastIf 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 mana value is 4. Shining Shoal also has an ability that says “You may exile a white card with mana value 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 mana value 2, then X is 2 while Shining Shoal is on the stack and its mana value is 4.