King Sombra was a First and Second Era unicorn king of the Crystal Empire. He excelled in the employment of dark magic, and was able to survive as a disembodied shadow for over a thousand years. Twilight Sparkle remarks that he was a "cruel despot" that held the Crystal Empire in "a perpetual state of slavery". In the First Era, he made a brief attempt to regain corporeal form, but was defeated. When encountered in the Never by Breakbeat and Lozenge, he is being forced to pull the Chariot of Fate (steered by the Blind Driver).
While in the Never, King Sombra teaches dark magic to Breakbeat, who he believes is innocent enough to be corrupted by it. He planned to use her as a stepping stone to return to the physical realm. His plan ultimately fails when she is killed by Prince Solstice, though the magic he taught her ends up being the catalyst that saves many ponies' lives.
Sirocco remarks in the epilogue that "None of us knew how close Sombra was, and he's still out there, in between everything."