King Sombra, a unicorn with a heart of no sympathy nor mercy was turned to shadow then was defeated when he rose a second time, But when he returns he has a different trick up his sleeve which endangers not only the crystal empire but the pony he targets to get out of his way to cause weakness to its new rulers. But with a recently recruited guard as a witness he may have an obstacle in his way.
(I give credit to TwoTonedSilver for the idea)