King Sombra was a unicorn and the main antagonist of "The Crystal Empire Part 1" and "2" the tyrant ruler of the Crystal Empire who enslaved the Crystal Ponies and brought hatred and fear to the empire.
1,000 years ago King Sombra took over the Crystal Empire and enslaved its inhabitents and brought hatred and fear to the empire. But not until Princess Celestia and Princess Luna used thier powers to turn Sombra into shadow and then banishing him to the ice of the Artic North. But not before he was able to put a curse upon the empire that caused it to vanish into thin air.
King Sombra is similar to the dark lord Sauron and the evil Sooty owl Surtr as they all are evil. King Sombra rules as a tyrant in the crystal empire, Sauron uses the One Ring to take over Middle-earth forever, and Surtr & the Pure Ones try to take over the owl kingdoms.