A mechanic from Generation 6 Pokémon X/Y games, this feature was released on PFQ on November 21st, 2013.
When you have the Mega Ring, you'll also get a random Mega Stone from Professor Holly and a section at the Merchant's will become available. Mega Evolution allows some fully-evolved Pokémon that are holding a certain Mega Stone to evolve once more. This evolution can either be temporary or permanent.
A Mega Stone is a special item needed for Mega Evolution. Each Mega Stone is specific and can only Mega Evolve a certain Pokémon. The stone contains the name of the Pokémon it can evolve in its name and summary. After Mega Evolving a Pokémon, the stone will break. You can buy Mega Stones from the Black Market.
The Mega Ring is a special Key Item that is the second component in unlocking Mega Evolution. Unlike other Key Items which can be bought in the shop, the Mega Ring only be unlocked by completed a special task. Spoiler - Task
Evolve Pokémon a total of 200 times or more, then visit Professor Holly.
How to Mega-Evolve a Pokémon
Visit Doug the Merchant with a Mega Ring. The last section, Mega Evolution, has a link to said page. Once there, you'll see a list of Pokémon in your Party. If you only have Eggs, it will tell you that there are no Pokémon. Select the Pokémon you wish to Mega Evolve. A warning shows that the Mega Stone will break in the process. Next, you can choose how long the Mega Evolution will last.
- 30 Days = Credits
- Forever = GP
Redeem a Voucher
If you have at least one Mega Stone Voucher, a new tab under 'Mega Evolution!' will be displayed, along with all the vouchers that you have in your inventory. To redeem a voucher, click on it. A button will appear in the tab for confirmation. There are two types of vouchers you can redeem : the Basic Voucher and the Plus Voucher. The Basic Voucher does not let you choose which Mega Stone you will receive, but instead chooses a random Mega Stone from the tier in the name of the voucher. The Plus Voucher allows you to choose any Mega Stone from a certain tier.