Once upon a time Sombra, the Dark King and Conqueror of the Crystal Empire, was an orphaned colt in the old capital of Xenophon. Adopted by Celestia and taught by the princess he becomes a powerful shadow mage.
Once upon a time Celestia, Princess of Equestria, heard stories of a orphan with a gift of magic. She adopts the colt Sombra as her son and student.
Shadows don't necessarily mean evil, but the path is dark and even the best taught of student can succumb to the taint of Darkness. Teacher and Student face each other for the fate of two nations. Sun and Shadow.