Celestial Kirin
Legendary Creature — Kirin Spirit
Flying
Whenever you cast a Spirit or Arcane spell, destroy all permanents with that spell's mana value.
All bow at its visitation—some in awe, some in honor, some in fear.
Adam Rex
Rulings
If the Spirit or Arcane spell has {X} in the mana cost, then you use the value of {X} on the stack. For example, Shining Shoal costs {X}{W}{W}. 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.
(wotc • 2005-06-01)
Rulings
If the Spirit or Arcane spell has {X} in the mana cost, then you use the value of {X} on the stack. For example, Shining Shoal costs {X}{W}{W}. 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.
(wotc • 2005-06-01)