Aether Vial
From the Vault: Relics — mythic
Artifact
Aether Vial rulings:
2017-11-17: If Aether Vial leaves the battlefield while its second ability is on the stack, use its last known number of charge counters to determine what you may put from your hand onto the battlefield.
2017-11-17: If a card in a player’s hand has in its mana cost, X is considered to be 0.