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Deals damage. Attack changes based on terrain. Becomes Earthquake in Wi-Fi battles.
A unique and very interesting move, Nature Power has a ton of potential in Wi-Fi battles and in the simulators, complimenting the movepool of several Pokemon that enjoy an Earthquake clone in their arsenal (and don't have access to the real one obviously). Unfortunately, the poor distribution shatters the dreams of many. A shame, indeed. That said, the only common Pokemon that can really make use of Nature Power are Shiftry and Sawsbuck, who enjoy the complimenting coverage that a powerful Ground-type move provides alongside their STAB Grass-type options. Most other Pokemon with access to this move either also have access to Earthquake or have no reason whatsoever to use this attack.
An interesting quirk to note about the move is that it bypasses the effect of Sucker Punch, which can make late-game sweeping with the aforementioned Pokemon easier if that's your opponents last form of priority.
Earthquake is the same exact thing in Wi-Fi and simulator battles.