The ancient necropolis of Nightmare moon has been awakened by her brief return to Equestria preceding her defeat. Now its abhorrent denizens are revived after a thousand years of debauchery and decay and with no direction or control. They will soon break free from their dungeons of night and spill across Equestria spreading darkness and death. That is, unless six friends, accompanied by the better part of the city's creator, can stop the spread of chaos and and gloom before all of Equestria falls victim to the things made in secret by an angry heart.