King Sombra was defeated by the power of the Crystal Heart. Some time later he was found severely weakened and was locked away. He would undoubtedly face execution, but he alone holds information on the Crystal Empire's past, and so Twilight Sparkle is tasked to learn what she can from the ex-tyrant. From him she'll learn the forgotten history of the empires creation and the reign of the crystal queen, as well as the story of Sombra's family and the mad king's rise to power.

(Sombra fic that also attempts to explain some of the peculiar elements of the Crystal Empire e.g. why they are mostly earth pony.)