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, Royal Decree deals 1 damage to that permanent's controller.
Official Rulings for Royal Decree
10/1/2008 : 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.)
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