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While a Pokémon is Mega Evolved, its Mega Energy will deplete over time. (Only one of your Pokémon can be Mega Evolved at any particular time.)
You can receive more Mega Energy for a particular species by battling that creature in Mega Raids
Mega Pokémon will boost the attack power of all other Pokémon when battling in raids
Mega Pokémon can be set as your buddy, though like Legendary creatures cannot be left to defend gyms. They can be used in battles against your friends but are (for now) not usable in the competitive Go Battle League. Shadow and Clone Pokémon cannot be Mega Evolved.
September will be filled with Mega Evolution-themed events, further details of which will be announced closer to the time. September 1st to 7th will involve battling in Mega Raids, while 11th to 17th September will task you to compete in gym, trainer and Team Rocket battles. Lastly, 22nd to 28th September will feature buddy tasks.
