This is my first theory i've ever put online so be nice :).

Mega evolution is a phenomenon that was introduced in Pokémon X&Y, it's a process that allows fully evolved Pokémon to extend past their evolutionary limit and become a new form with higher stats and a new type. Before a Pokemon can Mega evolve it must first have met certain requirements:

1. The Pokémon must be holding a compatible mega stone.
2. The trainer must be in possession of a key stone.
3. The Pokémon and the trainer must share a unique bond.

In the games mega evolution and mega stones seem to be given an aura of an ancient art, the tower of mastery in shalor city