Sombra is a Unicorn and previous ruler of the Crystal Empire before his defeat by Celestia and Luna. He made his return to retake control, but was defeat again by Twilight Sparkle, only to make a return once more, though this time changing targets to the Gems, only to be defeated once again. He currently is being imprisoned in Tartarus alongside Tirek. He is also the primary antagonist for Season 1.
He is fairly tall for a Unicorn, standing at about the same height as an Alicorn would be. His fur is a dark grey, and his mane and tail are pitch black, and wavy like shadows, stretching down the sides of his face like sideburns, with a red horn on his head. His eyes are red, and almost reptilian-like with slit irises, and turn green whenever he's doing his fear magic.
As a king, Sombra's outfit is that of a tyrantical ruler: a silver crown on his head with six different points, the smallest ones colored red. His neck and chest are covered in intimidating armor plates, and similar armor covers the front of his legs and his hooves. His back is covered in a red cape line with white, speckled fur.
Sombra is protrayed as a merciless, ruthless, and cruel tyrant with little care over his subjects other than to be used as pawns for his own personal gain. It means little to him exactly who he messes with, and enjoys messing with younger children more, since they're easier to fool and corrupt.
He is shown as calculative, and clever, able to create a distraction in Nightmare Moon in order to keep the princesses busy as he went with his plans on Earth, and give a convincing, fake report to Homeworld enough to convince The Diamond Authority to send Jasper down. However, he was able to recognize his defeat, and gave up quietly once they did beat him.
After studying the rules of Steven's world, he was able to figure out how to corrupt someone in terms of Corruption, and used it on Steven. It is unsure if it's the exact same form of corruption as it was with true corruption.
Sombra, as his namesake, is able to travel as a shadowy substance.
While more prominent in Gems, and more mental in Humans and Ponies, Sombra is capable of controlling those around him in terms of magic. In a Gem, he can make their bodies do what he wishes by controlling their Gemstones, and he can hypnotize Humans and Ponies in order to do his bidding and better fear him.
While unsure of his origin, Sombra became ruler of the Crystal Empire during his lifetime, ruling with an iron hoof until Celestia and Luna arrive and imprisoned him into the icy north. However, he laid a curse on the empire to make it disappear along with him, and when it return thousands of years later, so did he. He tried again to take over, but was defeated again by the Mane Six, this time getting his horn broken off and his body destroyed.
He came back for a third time however, and instead of targeting Equus, he went with the Crystal Gems to their world, and started to manipulate Steven Universe into falling into his trap, saying he had an answer to bring Rose Quartz to life again. This was all a ruse however, and he went on to corrupt Steven, much to the shock of the others. He reappears once again later on in the Crystal Temple, and placed a curse on Ruby and made her his slave, which he used to go back to Equestria, and turn Luna into Nightmare Moon as a distraction for Celestia so she wouldn't interfere. He found Peridot sometime later, and destroyed the Galaxy Warp for her, before giving a fake report back to Homeworld that Peridot was captured by the Crystal Gems, planning to board the ship coming in to go to Homeworld and take over.
He didn't plan, however, for the ship to be destroyed in the process, and captured Twilight Sparkle, Sapphire, and Lapis Lazuli, explaining his plan to them in a moment of triumph before bringing Sapphire outside for Ruby to shatter. This plan didn't work, and he soon decided to use Connie Maheswaran as bait instead to lure the Gems into the open, bringing out Steven and Lion. However, he was found by Peridot, and he tried to drive her away, admitting he will never comply to her demands. In the end, he was defeated when Peridot when her Limb Enhancers blasted his horn off, cracking it. Later he gave himself up, giving her the Portal Key before willingly going to Tartarus for his imprisonment.