Royal Decree is a Continuous Trap Card.
Trap Cards are normally Set first and can be activated on a later turn, often in response to an action.
On the current TCG and OCG Forbidden & Limited lists tracked here, Royal Decree is Unlimited.
YGOPRODeck lists Royal Decree for these formats: GOAT, OCG GOAT, Common Charity, Edison, TCG, OCG, Master Duel.
A listed print of Royal Decree appears in Dark Beginning 2 (DB2-EN106) at Super Rare rarity.
Among markets shown on this page, the lowest listed snapshot for Royal Decree is about $0.31 on Cardmarket (prices change frequently).
