Certain Pokémon can evolve into more powerful forms for a limited amount of time, and you'll soon be able to tap into this potential in Pokémon Go. Using a new resource called Mega Energy, specific Pokémon like a Venusaur, Charizard, and Blastoise will be able turn into a fourth form. You obtain this new resource by completing Mega Raids. The faster you can take down the boss, the more energy you'll get as a reward. Just like feeding Pokémon candies, you'll need a specific amount of energy to evolve into the new forms. Once a Pokémon reaches its Mega state, the energy will slowly decrease over time, and once it's gone, the creature returns to its previous state.
Only one Pokémon can be Mega evolved at once, and it can't be a shadow or clone. These powerful forms are great for battles, but can also do every other activity in the game, including being your pet. If you are trying to "catch them all," the Mega Evolutions are catalogued in a new Mega Pokémon, so you'll need to evolve each Pokémon at least once. This big addition to the game also comes with new special research mission called "A Mega Discovery," which concludes with trainers getting the chance to unearth a Mega Beedrill.
Each week in September brings a different Mega event. We don't know how many Mega Evolutions will be available out of the gate, but this is another area that Niantic can spread out over the course of the year, something the team has been excelling at over the last couple of years.