Royal Decree — ruling 1

Royal Decree’s ability will trigger a maximum of once for each permanent that becomes tapped, even if that permanent meets multiple criteria. (For example, Badlands is both a Swamp and a Mountain.)

Royal Decree : règles, fonctionnement et interactions (MTG)

Royal Decree is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Cumulative upkeep (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.) Whenever a Swamp, Mountain, black permanent, or red permanent becomes tapped, this enchantment deals 1 damage to that permanent's controller.

How it works : Royal Decree

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2008· Wizards of the Coast
Royal Decree’s ability will trigger a maximum of once for each permanent that becomes tapped, even if that permanent meets multiple criteria. (For example, Badlands is both a Swamp and a Mountain.)
No other card is named in this ruling.