The Kalos region introduced Mega Evolution in the main Pokémon series.
Mega Evolution debuted in Pokémon X and Y, the sixth-generation games released for Nintendo 3DS in 2013. It is a temporary battle transformation that allows certain fully evolved Pokémon to change form and gain increased power. A Pokémon generally needs its corresponding Mega Stone, while the Trainer needs a Key Stone.
Kalos is inspired primarily by France, with Lumiose City drawing on Paris. The games connected the new battle mechanic to themes of beauty, bonds, and evolution. The temporary nature of Mega Evolution also helped preserve balance: it was powerful, but it did not permanently replace a Pokémon’s ordinary form.
Later games expanded the mechanic with additional Mega Evolutions. Pokémon Omega Ruby and Alpha Sapphire introduced further forms, including Primal Reversion for Groudon and Kyogre. That follow-up can cause confusion, but the original introduction belongs to the Kalos-set Pokémon X and Y.