Aether Vial

Oracle Text

At the beginning of your upkeep, you may put a charge counter on Aether Vial.

T: You may put a creature card with converted mana cost equal to the number of charge counters on Aether Vial from your hand onto the battlefield.

Card Rulings

11/17/2017 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.
11/17/2017 If a card in a player’s hand has in its mana cost, X is considered to be 0.