For the briefest of moments, Anonymous wielded absolute power. He single-handedly destroyed the city of Canterlot, laid waste to Ponyville, decimated the Everfree Forest, and came dangerously close to annihilating the very Tree of Harmony itself. And he had an absolutely fantastic time doing it.
Right up until he lost.
Now he's stuck as a statue under the watch of the princesses he tried to subjugate, awaiting the time he'll be locked away forever, with the key no doubt thrown into a volcano. But from the very beginning, Anonymous suspected that the shenanigans of the good guys would cost him his victory at the last moment, and so he laid plans with his most loyal of subjects—all former soldiers of the Storm King he betrayed to obtain his power—to arrange for his release should the opportunity arise.
Unfortunately, he wasn't counting on the ponies calling in help from a faraway land. Enter the Four Princes of Zebrica, all wielding the same exact power that Anonymous did, and knowing a thing or two about how to put bad apples like him away for good. This is made even worse by the sorry state of Anonymous's storm beast forces, having hit hard times in the months since his defeat.
But not all is lost for the petrified supervillain. Because in a rare twist of luck, the Princes just so happen to unknowingly bring with them a zebra criminal with his own dirty little secret. A secret that not only secures Anonymous's release from captivity, but just might be the start of a horrifying new partnership, and with it, the birth of a new force of evil.
And of course, with all that going on, there's always the matter of the nascent Legion of Doom, plotting to bring their own age of darkness to Equestria as well...

Takes place during the final two seasons of Friendship Is Magic. With a few alterations here and there, because I HATE THE ANTI-FIM I HATE THE ANTI-FIM :bread:
Reading Rider on the Storm is strongly advised if you want any idea of what the hell is happening in this story.