Infernal Kirin — ruling 1

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.

Infernal Kirin : règles, fonctionnement et interactions (MTG)

Infernal Kirin is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever you cast a Spirit or Arcane spell, target player reveals their hand and discards all cards with that spell's mana value.

How it works : Infernal Kirin

Original English text — Wizards of the Coast / Scryfall

#1June 1, 2005· Wizards of the Coast
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 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.
No other card is named in this ruling.