Thranduil's Decree
Counter target spell. If a permanent spell is countered this way, exile it instead of putting it into its owner's graveyard. You may cast that card without paying its mana cost for as long as it remains exiled.
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Official rulings (3) · Scryfall
You must still follow any timing restrictions and permissions, including those based on the card's type.
If the card has {X} in its mana cost, you must choose 0 as the value for X when casting it without paying its mana cost.
Because you're already casting the card using an alternative cost (by casting it without paying its mana cost), you can't pay any other alternative costs for the card, including casting it face down using the morph ability. You can pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.