Finale of Devastation
Search your library and/or graveyard for a creature card with mana value X or less and put it onto the battlefield. If you search your library this way, shuffle. If X is 10 or more, creatures you control get +X/+X and gain haste until end of turn.
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Official rulings (4) · Scryfall
No player may take action between the time you reveal which creature card you'll put onto the battlefield and the time it gets +X/+X and haste if X is 10 or more. Any abilities that trigger as it enters the battlefield will be put onto the stack after your creatures get +X/+X and haste.
If you don't find a creature card with mana value X or less, creatures you control still get +X/+X and gain haste if X is 10 or more.
If X is 10 or more, the creature card you just put onto the battlefield will get +X/+X and haste.
If a creature card in your library or graveyard has {X} in its mana cost, X is considered to be 0.