Celestia's guards, calm, unmmoving, showing no signs of emotion, it's almost like they've had their hearts closed. Actually that last part's true. As part of their training they have a spell cast upon them that closes their hearts and makes them loyal only to Celestia, what ponies don't know is that it also grants powers similar to Nightmare Moon. Only in history have 2 ponies ever resisted the spell, one has his heart partially closed making him a loose cannon for the dark powers, the other is blissfully unaware and remains loyal to Celestia. Now the single guard must survive the odds and attempt to find a way to cure his condition without getting those around him killed.