The Scouring

The Scouring (0-7 AS) was a series of surges of wild magic that caused widespread destruction and upheaval amongst the races of Equus.

Magical Mayhem
Wild magic wracked Equus, throwing the world into a dark age. Portals also opened to many of the Outer Planes, making conjuration once more possible, as well as bringing new monsters to Equus. The Princesses Celestia, Luna, and Cadance sacrificed themselves to bind the wild magic and send it away from this world, hopefully forever... However, the damage was done and many monsters had been brought to the realm, unable to return.

Cause
The wild magic, unbeknownst to ponykind, were caused by a battle that flew across the planes between two extremely powerful, yet unknown, magi in a duel that ended only when the archmagi used their strongest magics, beyond the ken of any mortal magic user. The spells caused tears in the fabric of reality. After the battle, neither combatant knew what they had done, having continued on with their battle on other planes, thus leaving the problem for the inhabitants of the world.

Effects
Due to the effects of the wild magic, the barriers that prevented all races from wielding magic dissipated and magic was let loose for all races. The political stability of the former Ages was gone, the Griffon Empire was the first to capitalize on the devastation. Later during the Scouring, an Earth Pony named Helping Heart became the first priestess in a thousand years. With the resurgence of clerical magic, arcane magic was given a second look, many spellbooks from the Ages of Chaos and Harmony being found in the archives of many cities, such as Canterlot.