Vivien's Arkbow
Legendary Artifact
, 
, Discard a card: Look at the top X cards of your library. You may put a creature card with mana value X or less from among them onto the battlefield. Put the rest on the bottom of your library in a random order.
Zack Stella
War of the Spark Promos (PWAR)
• #181sRulings
If a card in your library has {X} in its mana cost, X is considered to be 0.
(wotc • 2019-05-03)
Rulings
If a card in your library has {X} in its mana cost, X is considered to be 0.
(wotc • 2019-05-03)