Twilight Sparkle is a very gifted unicorn filly. At her entrance exam for Celestia's school, she sees a flash of the future, a vision, a spell, and it bursts from her horn unwittingly, rippling across Equestria at the same time as the Sonic Rainboom, unknowingly altering destiny for five other ponies. Years later, when she is told to move to Ponyville, she realizes something is terribly wrong.

Luna, after realizing she's slipping back into the attitudes of Nightmare Moon, creates the Tantabus. However, it comes with unintended consequences...

Like a new alicorn.