King Sombra never enslaved the Crystal Empire. Instead, he chose to join his best friend, Radiant Hope, in Equestria while she studied under the Royal Sisters. With the sins of his past haunting him, Sombra decides to spend his time training to face whatever Hope will as she ascends the Throne.

(This is based off of FIENDship is Magic #1 First chapter contains a rundown of the events of the comic, save for the end."