The Legendary Pokémon Mewtwo has finally arrived in Pokémon Go, with Niantic announcing Mewtwo’s appearance at the Pokémon Go Stadium event in Yokohama, Japan, where thousands of Trainers had the chance to capture it. Following this appearance, Trainers worldwide will also be able to battle and catch Mewtwo as part of the new Exclusive Raid Battle feature.
Exclusive Raid Battles differ from regular Raid Battles in that Trainers have to be invited to join them. In order to be invited to an Exclusive Raid Battle, you must first have successfully completed a recent raid, defeating the Raid Boss, at the Gym where the Exclusive Raid Battle is being held. The invites will also alert Trainers in advance to when the Exclusive Raid will be talking place, so that they can plan to take down the Raid Boss with other Trainers.
Mewtwo will be the first Pokémon to star in the Exclusive Raid Battles, and then other powerful Pokémon will start to hatch from the Raid Eggs that you can find at Exclusive Raid Battles.
Niantic also announced that from August 14 until August 31, Legendary Pokémon Articuno, Zapdos, Moltres, and Lugia will all be appearing in Raid Battles.
Source: Mewtwo Coming to Exclusive Raid Battles Soon! - Pokémon GO
