And is also a sequel to...

"The Pirate Pegasus",

"The Pony Weapon",

"Rise of Empress Midnight",

"Seeking the Legend",

and, "The Tragedy of Queen Chrysalis"

Don't worry if you haven't read all of these stories, the prologue has a recap.

Two months after the events of "The Adventures of Arcega Minn", Twilight and her friends go to the museum in Canterlot to attend to opening of the Heroes Wing. However, during the opening ceremony, things take a turn for the absolute worst.

Sanctus Nightmare, the God of Nightmares, has returned once more, but he isn't alone. At his side are past villains who have come for vengence against Twilight Sparkle and her friends. Captain Korsan, the mechanical pirate, Empress Midnight, the multidimensional warrior, and Queen Chrysalis, the Changeling ex-Queen, all revamped and ready to strike once more.

It will take the efforts of everypony to stop them before they plunge the entire universe into darkness. Friendships will be tested, alliances will be torn to shreds, and there's no knowing who will come out of this the same as they were before.