Nature Power becomes a different move depending on the environment that the battle is being conducted in.
Nature Power targets the user rather than any specific Pokémon. Therefore, in double battles, because of the inability to select a target, the chosen move always hit the opponent directly opposite the user. If the opponent directly opposite the user had fainted, the move will randomly target the other opponent. If in a terrain that calls a move that affects multiple Pokémon, it will target the Pokémon usually targeted by that move.