When the captain of the dark guard is shrouded in mystery and his own elite group of soldiers are scared into perfect obidience it is time to check who exactly your employing. Shadow Moon is under oath to protect Luna and command the Dark Guard under her will, but only ordering his troops through telekinesis and having no mercy on anyone he does it in a unpredictable, cold and calculated manner. So when Nightmare Moon rises he lives up to his oath and protects her up until clashing with Celestia were she is banished and he has to change his oath of servance to fit this certain problem.

This description sounds like a setup up for a romance comedy but if you want that check out My Blue Love. This is dark and gory most of the way though.

There is a slight anthropmorphic bent added in later.