Character: king sombra
King Sombra is a unicorn described by Princess Celestia as having "a heart as black as night." He enslaved the ponies of the Crystal Empire over a thousand years ago, and ruled them as a tyrant until he was struck down by Princess Celestia and Princess Luna. He was transformed into living shadow and locked away within the arctic ice, but not before he cursed the Crystal Empire to vanish.
More recently, the Crystal Empire returned, and with it King Sombra. He manifested first as
a vast cloud of darkness the smoke monster from Lost, later regaining his equine head which allowed him to say things like "CRYSSSSTALSSSS" and "SLAAAAAAVESSSS," and finally reclaiming his physical form. He was almost immediately blasted to fragments through the power of the crystal heart, though it is uncertain whether this killed him.
King Sombra is a dark gray unicorn with black hair and fangs. His eyes are red with green "whites," and trail violet light like fire; this may be related to the unusual type of magic he wields (manifested briefly by both Princess Celestia and Twilight Sparkle). His horn is red and curved, and lacks the spiral pattern common to unicorn horns. He wears a metal crown with curving spikes that resemble his own horn, and is also shown wearing armor and a rich cloak.
In the IDW comic arc "Reflections" The nice-guy parallel universe version of Sombra had a thing going with Prime Universe Celestia. It fell apart when both universes began collapsing from all of the cross-dimensional travel. Good-Goy Sombra absorbed the evil from Mirror Celestia and Mirror Luna and subsequently became as evil as his Prime Universe counterpart.
Meghan McCarthy is on record saying that Sombra did not turn out to be nearly as intimidating as they had hoped that he would be. But what the hey. The show provides so few decent villains that we will take whatever we can get.