The Other Elements (mlp:fim fanfic)

When Equestria's safety is threatened by the dark forces of Queen Chrysalis, King Sombra, and Tirek, Princess Celestia enlists Twilight Sparkle and her friends to aid in the protection of Equestria. But with the Elements of Harmony back in the Tree of Harmony, a new source must be used to keep the enemy at bay: The El...