There were five alicorns in the prophecy Star Swirl decoded. "Two sisters, night and day. One Alicorn to represent the Crystal Kingdom. An Alicorn not born, but one made. And one to be a mystery." The first four are found. But where's the fifth? Celestia's discovered something new-that Star Swirl didn't finish decoding the prophecy before he died. And when Celestia tries to decode the rest-she finds out something horrible about this new fifth alicorn.
And there's even more to the prophecy that Celestia can't decode...