King Sombra wasn't always a black-hearted unicorn. He was once one of Princess Celestia's most faithful students, a young unicorn preceeding Twilight Sparkle by over a thousand years. He had shown promise and high aptitude just like Twilight Sparkle has.

However, after graduation, his path took a decidedly darker turn than Twilight, leading up to the events that took place just prior to Nightmare Moon's banishment.

(Yes, another King Sombra story)