Mega Evolution is a transformation that certain Pokémon can undergo while holding a compatible Mega Stone, provided their trainer also has a Mega Bracelet. This transformation changes the Pokémon's appearance and increases its stats, and may also change types and abilities. Mega Evolution can be used outside of battle through an external move, Mega Evolve.
To be able to use Mega Evolution, a player must have an equipped Mega Bracelet (obtained from winning your first fight with a Mega Evolved boss Pokémon) and a Mega Stone compatible with the chosen party Pokémon. Please note that bracelets cannot currently be spawned in through Creative.
Mega Stones can be obtained from their corresponding Mega Evolved boss Pokémon. The first defeated Mega boss of each Pokémon species will always drop its Mega Stone, while subsequent defeats will have a 1 chance of dropping another copy of the stone.
This means that when you beat your first boss, you will always receive a copy of its stone and a fresh Mega Bracelet. Subsequent defeats of the same boss will have a 1 chance of dropping another copy, but other bosses are still guaranteed to give you their stone until you've beaten them too.
Once all requirements for Mega Evolution are met, a Pokémon can be Mega Evolved whenever it is holding its Mega Stone. Please note that the player must be wearing the Mega Bracelet; Mega Evolution will be unavailable if is it not equipped.
Before the Pokémon uses a move, there will be an option to Mega Evolve. If this option is selected, the Pokémon will Mega Evolve immediately, after which it will be able to pick a move like normal. Please note that Speed updates on the next turn; turn order is decided before Mega Evolution.
Mega Evolution persists throughout the battle, even when the Mega Evolved Pokémon switches out. The Pokémon will revert back to its base form if the battle ends or the Pokémon faints. Only one Pokémon per party may Mega Evolve during a battle, with others automatically getting locked out.
In-battle Mega Evolution can be prohibited by enabling the Mega Clause battle rule. This will also prohibit the use of any Pokémon that are holding their Mega Stones.
NPC Trainers do not require a Mega Bracelet to use Mega Evolution, but still require a Mega Stone to be held by a compatible Pokémon. NPC Trainers in higher-leveled Gyms may occasionally use Pokémon that can Mega Evolve.
Outside of battle
When out of battle, Mega Evolution can be initiated by selecting the Mega Evolve external move. This will only work if the player has defeated a Mega boss of the same exact type as the Pokémon they're trying to evolve.
For more info, see the main article at: Mega Evolve (external move)