Harmony is Magic
Following the Siege of Tirek and the Arrival of the Castle of Friendship, Equestria fell into a time of peace. The Princess of Friendship and the Elements of Harmony embraced their new positions and led Ponyville to become Briddleton, a thriving city rivaling that of Canterlot. After great consideration, Princess Twilight Sparkle decreed that a grand hall of knowledge must be constructed in the heart of Briddleton to replace the one lost in the Siege. It was called the Library of Harmony after its founders. Remembering fondly the days she spent under Celestia's tutelage, she announced that she would be taking on an apprentice. After months of reviewing applications and sitting in on interviews with candidates for her School of Magic, she chose two young unicorns, Moonlily and Inkwell. The third, a young Earth pony named Star Serenity, was found deep in the Library of Harmony surrounded by stacks of spell books. Twilight offered her a chance to learn magic under her guidance and Star accepted.