A new mage-pony comes to ponyville, he's rather old and he's an earth pony, he is the most powerful Mage Twilight has ever seen. Then she gets message from Celestia saying that he is too dangerous and the six should use elements to deal with him. But Twilight and others had already become friends with a mage. They ask him why Celestia is so afraid of him and he tells the story about his youth explaining the issue. ( I don't know, maybe he was to determinated to overcome others and his deeds were too cruel and dangerous ). Then Twilight shows Celestia that there is nothing bad the oldpony can do to Equestria, the Mage makes an apology to Celestia and she responds that she has learned a lesson about the bad guys may change and become great friends even to old enemies. ( Sorry for my English, not native speaker).