The day King Sombra was defeated became a holiday for the Crystal Kingdom. It was named "Crystallization". However, he was never actually killed at the hands of Twilight Sparkle, Shining Armor, and Princess Cadance. His physical form was simply destroyed, but his spirit, will, and soul were still intact. He fled into the shadows of the world and awaited silently for his return. He rose an army of shadows that take form corpses he so chooses, or at random when he doesn't decide. Sombra eventually left the shadows with connections he had from the darker part of the world.