While exploring the Canterlot libraries, Twilight and Spike find a mysterious book titled "The Chaos Diaries". Upon reading it, they learn just what it was like living under the tyrannical and chaotic rule of Discord many hundreds of years ago when he first appeared in Equestria, and how exactly his reign on terror came to an end.

Darkness falls on Equestria as an enemy long thought dead returns to claim the throne and turn Equestria into a nation of war. When a mysterious dark unicorn arrives in Ponyville seeking shelter, the Mane Six find themselves caught up in a battle the likes of which they have never seen. What connection does this dark pony have with the rising threat? Can Twilight and her friends win the fight when even Celestia and Luna fall? What secrets has Celestia been hiding from her subjects? Can a sonic rainboom outpace a living lightning bolt? All of this and more will be answered when Equestria comes under a new reign. A reign of fire.