Princess Twilight has the duty of opening the new Equestria History Museum in Canterlot, an event that has drawn many important ponies and creatures from across the globe. The event being of such importance, that Rarity felt fit to create a whole new dress for Twilight. All she needed was a gemstone fit for a Princess.

However, when searching in the mines for the perfect gem, the Gem of Tartarus. Unbeknownst to Rarity, whoever wears the gem will be possessed by a power of utmost evil. Can the Mane 6 stop the evil power before it can cause havoc?