King Sombra

Here is the evil and dark

King Sombra Mahahahahaha!

is a male unicorn, the former tyrant ruler of the Crystal Empire, and the main antagonist of the season three premiere. Princess Celestia tells Twilight Sparkle that he took over the Crystal Empire a thousand years ago, but was banished into the frozen north. King Sombra put a curse on the Crystal Empire which made it vanish into thin air. When the Empire returns, so does King Sombra, who attempts to retake it. He is defeated by the Crystal Ponies' use of the Crystal Heart. His dialogue consists of growls, snarling, hissing, menacing laughter, and five lines.
His personality is described by Princess Celestia as a unicorn whose heart is "black as night". In terms of personality, King Sombra is shown to be sadistic, fierce, and cruel, as evidenced by his referring to the Crystal Ponies as his "crystal slaves" and appearing to take delight in seeing them suffer. Though rarely speaking, he appears desperate to retake the Crystal Empire, as he chases Shining Armor and the Mane Six during their first encounter and cursing Shining Armor, causing dark crystals to grow on his horn and nullify his magic, and when rushing toward Spike in an attempt to regain the Crystal Heart.
But in the My little pony comics his personialty is a parallel version of King Sombra in another world is stated by Celestia to rule that world's Equestria with "kindness and benevolence", as opposed to the King Sombra seen in the series. In his introduction he is shown to be generally very kind and polite. He is also quite dashing (as evidenced by Rarity and Fluttershy swooning over him) and charming. The alternate Sombra is later revealed to be deeply in love with Celestia, who reciprocates his feelings.