The Beast of Vengeance

In the Spirit Realm Master Celestia is meditating peacefully when she is attacked by a Spirit Warrior: a horned beast named Tirek who fought against Celestia five hundred years previously. The two battle, and Celestia learns that Tirek has increased his strength by stealing the Chi of every creature in the Spirit Realm, and only needs Celestia's to become strong enough to return to the mortal realm. Claiming victory, Tirek is nevertheless warned that Celestia was never Destined to defeat him, but that she has set someone else on that path. Meanwhile, in the Ponyville, Twilight and the Furious Five learn that Luna will no longer be responsible for overseeing their training: that responsibility now falls to Twilight.