The Everfree was once home to the Castle of the Royal Sisters. The alicorns of both Sun and Moon, who moved the days and nights.
But then, Luna, The Moon Maiden, the Dreamwalker, became jealous of her sister's ever casting shadow. In a fit of rage, she became the terror that is Nightmare Moon, a beast of darkness and hate. She challenged her sister, the Sun Sylph Celestia, to combat like no other. For when alicorns clash, none shall remain unchanged.
The battle was short, but legendary. Both pushed each to the limit, but it appeared that Celestia would fall. In desperation, she used the Elements of Harmony, the artifacts she and her sister had used to subdue the Chaos Bringer long before, to banish her sister to her own place of power.
But both the battle and the release of the Elements took their tole on the surrounding areas. Though it was small at first, it soon spread. ANimals went feral. Monsters began to roam. The sky moved on its own. Eventually, the ponies had to flee, abandoning the castle to build Canterlot in the side of a nearby mountain.
This breaking of the magical energies that hold our world together has been spreading, little by little, for over a thousand years. Though none have taken action to stop this spread, there is a growing fear, as the wound in the world now reaches its tendril towards the sleepy town of Ponyville.