King Sombra is a dark-gray unicorn with a black mane. He is an evil usurper armed with a heart "black as night" and a proficiency in dark magic, which often manifests as dark crystals. A thousand years ago, he conquered the Crystal Empire and enslaved its populace. He was later banished within the Frozen North, but not before he placed a curse that caused the Crystal Empire to disappear up until the present day. When the Crystal Empire reappeared, Sombra emerged from his banishment in a smokey, ghostly form and tried to retake the Empire, but was cast to the Aether by Princess Cadence and her subjects via the power of the Crystal Heart. He was later restored by Grogar for his bid to conquer Equestria, but Sombra haughtily began his own campaign to take Equestria for himself, only to be dispelled once more by the Mane Six and become an example to those who don't stick to Grogar's plan. As shown in "The Cutie Re-mark", there exists a timeline where he retakes the Crystal Empire unopposed following its initial reappearance and conquers a good chunk of Equestria afterward, forcing Princess Celestia and her subjects into an ongoing war with his mind-controlled forces.
As an ethereal, incomplete spectre in "The Crystal Empire", he is voiced by Big Jim Miller. After he is revived as a whole, corporeal being in "The Beginning of the End", he is voiced by Alvin Sanders.