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.(Based off FIENDship is Magic #1)
Sombra's final mad thoughts before encountering the pony sisters, his reaction to Twilight 1000 years later, and how the sisters (and Radiant Hope) mourned the Crystal Empire. A companion story to the recent "Fiendship is Magic" comic about Sombra.