This story is a sequel to Tides of the Past

Luna, princess and dream walker, is asked by Celestia to keep an eye on one Grimoire Maleficarum, a practitioner of Dark Magic and newly confined pony under Twilight's watch. To do in secret, she must enter the dreams of the pony and mine his mind to see if he is a threat to Equestria. Question is, what does she find?
(Technically following the events of Tides of the Past, it isn't necessary to this story)