If you’re looking to take your battles to the next level, register now for the Mega Melee Online Competition. Create a team using only Pokémon that can Mega Evolve and then let the battles begin! Remember that only one of your Pokémon will enter each battle. Duplicate Pokémon and items are not permitted. And every Pokémon will be set to Lv. 50 for the duration of the battles. The complete list of eligible Pokémon is available in the competition regulations. To register, log in to your account from the Pokémon Global Link (PGL) homepage and select Online Competitions in the main menu.
You’ll then be able to read and accept the regulations on the PGL. You’ll need a Pokémon Trainer Club account, your own Pokémon Sun or Pokémon Moon video game, and a Battle Team of eligible Pokémon. Accept the regulations on the Pokémon Global Link by Thursday, September 21, at 23:59 UTC, and get ready to battle! The competition goes from Friday, September 22, at 00:00 UTC, until Sunday, September 24, at 23:59 UTC. All qualified participants in the Mega Melee competition will receive 50 Battle Points. Players must complete at least one battle to qualify.
Source: Pokémon Global Link