King Sombra: a pony who was not a pony at all, but an Umbrum, that took control over the Crystal Empire in hatred—only to be banished as a shade under the ice by Celestia and Luna. He did put a curse on the Crystal Empire to ensure his return—but his plans were foiled once again, resulting in his first "death." Once more, he returned to plague Equestria, almost conquering it entirely—only to be stopped by Twilight Sparkle and her friends, and ultimately killed. Thus, his second "death."
That is what most know of Sombra. They view him as a monster of evil—but even monsters have regrets and memories of those they loved. Being held in an endless void gives one time to think about these things...
The awesome cover art is by Caeledonian on DeviantArt. I do not claim to own their artwork in any way. Source: https://www.deviantart.com/caeledonian/art/Somber-Sombra-401022852