Cloudhoof Kirin

Legendary Creature - Kirin Spirit 4/4, 3UU (5)
Flying
Whenever you cast a Spirit or Arcane spell, you may put the top X cards of target player's library into his or her graveyard, where X is that spell's converted mana cost.¹
Illus. Randy Gallegos

Gatherer Card Rulings [Nov 2009]

• 6/1/2005: 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 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.

Format Legality	Legal in Vintage, Legacy, Extended Banned in Standard
Printings (1/8)	Saviors of Kamigawa (Rare)