Mega Evolution

Mega Evolution is a special kind of Evolution. It makes an Elemental grow in size and power and changes their appearance temporarily.

Physiology
An ancient phenomenon that triggers an evolution of Elementals who actually aren't able to evolve anymore.

The Mega Evolution makes an Elemental grow in size and power immensely but at the same time might make those not used to it lose control. One common feared side effect is turning into a feral, blood-thirsty beast, not able to tell apart friend and enemy.

Mega Evolution is triggered with a corresponding mega stone for each species. The Mega stones have grown naturally over thousands of years.

Every species only has one stone, and many have been destroyed or repurposed over the years.

Trivia

 * Reshiram's Mega Stone was forged by destroying and merging 10 different species' Mega Stones. Since Reshiram, Zekrom and Kyurem were man-made species, they don't have naturally occurring Mega Stones.
 * Using Mega Stones is frowned upon, since their usage is incredibly dangerous.
 * Mega Evolving too much can permanently damage an Elemental's brain if overused.