While the endless hours of running a country can tire out even the most optimistic of alicorns, Twilight's new role as sole ruler of Equestria does have one benefit.

Unrestricted access to the Restricted Section of the Canterlot Archives.

One day, while reading a book shes' never seen before, Twilight accidently releases the spirit of the dragoness bound within its pages.

Unwilling to let the dragoness, Cinder, moulder away eternity trapped in a book, Twilight lets her latch onto her own self and share a body with her.

This leads Twilight to discover more of dragons than she ever thought possible.