Mega Evolution

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

Physiology
An ancient phenomenon that triggers an extra evolution on Elementals, that usually cannot evolve anymore. The Mega Evolution makes an Elemental grow in size and power immensely, but at the same time damages their brain, so that in many cases they lose control over their body and become a feral, blood-thirsty beast. Because of that, Mega Evolution is mostly frowned upon and not seen a lot.

Mega Evolution is triggered with corresponding mega stones 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.