Many centuries ago, a mysterious, demonic entity know as Sanctus Nightmare roamed the planet. He was incredibly powerful, but had vile intentions for the ponies of the world. He was eventually banished by an unknown force to the Void, an area between alternate worlds and timelines. But before being banished however, he left behind many remnants, parts of Sanctus Nightmare's body and soul, that connected him to the physical world from the Void.
One of these artifacts, named Margáil Le Bás, is a magical sword, capable of shape-shifting into various objects. At one point in time, it was transformed into the Mad Book of Power, an almanac that, when read, linked the main body of Sanctus Nightmare to the reader's mind. When read completely, it opens a portal to the Void, in which Sanctus Nightmare can go through and return. It was left behind in an abandoned church on the western continent of Wisteria, waiting to be opened.
Many years later, in the land of Equestria, King Solaris and Queen Ecliptic ruled over the land in an era of peace. Their daughters, Celestia and Luna, were being taught by many teachers and wizards on magic, science, and diplomacy.
Throughout this time, Princess Celestia and Princess Luna were educated under the servitude of many teachers, including Star Swirl the Bearded. They learned powerful magic, friendship, and the history of the chaotic land that was once "Equestria".
Princess Celestia developed a close relationship with one of the military soldiers, Relv. Orphaned early in life, Relv had few friends and little education, but his good heart managed to win hers. The two spent lots of time together, eventually resulting in Relv being bumped up to her personal bodyguard, further tightening their bond.
Celestia taught Relv what she learned during her classes, and the two would frequently mess with science equipment and do experiments. One of these experiments resulted in the creation of a substance, which amplified Relv's strength, but gave Celestia osteoporosis. Volunteering himself, and only himself, Relv proceeded to be injected with various altercations of the original substance, giving him powers even beyond known magic.
But then, disaster struck. Relv turned on Celestia and her family, becoming a murderous vigilante. He killed both good and bad ponies, and even got into several fights with Celestia. Eventually, this climaxed in the death of King Solaris and Queen Ecliptic. Relv was subsequently locked in the Interdimensional Containment Unit for Treacherous Enemies, a superprison in a pocket dimension.
Relv was to be kept there till the day he died, even if it took thousands of years.
Roughly 250 years later, the Mad Book of Power is discovered by Prince Thanatus, who is subsequently possessed by Sanctus Nightmare. However, he didn't finish the book, and he remained a proxy to Sanctus Nightmare under the name of Lord Thanatus. Sanctus Nightmare used him to invade kingdoms across the world and destroy them.
In the kingdom of Centura, a mare by the name of Arcega Minn overhears that the next kingdom Thanatus is going to strike is hers. Alongside her friend, Ebony Diamond, the two mares set off to stop him. With the aid of a bard named Silver Chord, a mercenary named Huntsmen, and a mysterious unicorn named Eclipse, they located Glacadh An Bháis, an indestructable sword.
On their way to obtain it though, Lord Thanatus, and furthermore, Sanctus Nightmare, possessed Eclipse and caused her to turn on the others. All of them, including Lord Thanatus, died, and his sword, Margáil Le Bás, was turned into a book titled "The Adventures of Arcega Minn".
Another 250 years later, and a new scene has entered the mix. Enter Crystal, princess to a kingdom of Trihearth Island, a small island split between three rival kingdoms. Set to wed the prince and love of another kingdom and bring peace to the quarreling lands, Crystal could not be happier. However, things turned for the worst, when a prince from the third kingdom made a bargain with a witch, setting off a terrible chain of events.
In true, poetic tragedy, Crystal drowned before the wedding, and is transformed into a Changeling proxy by Aspidochelone, a monster whose shell is the island of Trihearth. She is returned to the surface and was immediately betrayed by her lover, causing her to snap and adopt the identity of Queen Chrysalis.
The island disappeared into the ocean shortly after, and in its wake came Queen Chrysalis and her army of Changelings.
500 years later, the main events of My Little Pony kick off, with Twilight and her best friends coming together and defeating Nightmare Moon, stopping Queen Chrysalis, and reforming Discord.
After their fight with Lord Tirek at the end of Season 4, a portal opened to the Midnight Universe, a bleak, warring version of their world, all under the tyrannical rule of Empress Midnight. She invaded this world, and the Mane 6 are forced into a resistance lead by their inter-dimensional doubles. They succeed in defeating Empress Midnight, but at a cost. The Midnight Universe was destroyed, and Empress Midnight, having recently discovered to be Twilight's evil counterpart, was sent to a white void where her world once was. It was unknown if she could escape or even if she survived. But if she did, her only goal would be to have her revenge.
While Twilight recovered, Applejack went on adventure to the desert west, in order to prove or disprove the existence of The Ghost Pony. She nearly died, but was saved by a mysterious pony, clad in black, who referred to himself as Eldur. Applejack returned home enlightened, but hasn't confirmed The Ghost Pony's existence.
Fast forward past Season 5 and Starlight Glimmer's reformation, and old foes had returned to Equestria. Relv escaped from his prison pocket dimension, but is recaptured by Celestia. She then put a mind control spell on him and gave him to Fluttershy in the hopes that she can reform him. Fluttershy however bonded with Relv and freed him of the spell. Maddened by what Celestia did to him, he tried to kill her, only to be stopped by Fluttershy and her friends.
Immediately after that, a gigantic monster appeared and attacked Ponyville. Relv defeated the beast by tapping into his final, most powerful ability, becoming Nightmare Relv and transforming his body into an equally massive beast. Upon returning to normal, Relv left Ponyville and everypony, becoming the vigilante anti-hero known as the Black Paladin.
Months later, the Mane 6 went to the annual Grand Galloping Gala at the castle. But the party is cut short, as a mechanically augmented pirate named Captain Korsan, and his crew of one hundred unicorn, invaded the castle. He stole a priceless artifact from the gala, which turned out to be part of a map leading to Alicorn Island, an island which is said to house a means to becoming an alicorn.
The Mane 6 followed Korsan across the seas, befriending ponies and facing perils along the way. At one point, Rarity was captured by Korsan, and the two eventually developed a close relationship with one another. The two eventually split up though, inadvertently causing Korsan to lose a great deal of his already dying sanity. The Mane 6 stop Korsan in the end with the aid of his crew members seeking freedom of his control. Korsan survived their final battle despite all odds with help from his new alicorn form, and now seeks revenge.
After the nautical adventure, the events of Season 6 played out. Queen Chrysalis returned to Equestria, but is quickly foiled by the ponies and her Changeling army was taken away. Now wandering the world alone, Queen Chrysalis also seeks revenge against the ponies who've foiled her.
Long after Chrysalis's second defeat, Twilight found a book in the old Castle of Two Sisters. This book was actually Margáil Le Bás, and Sanctus Nightmare immediately took hold of Twilight mind. He possessed her, turning her against her friends and destroying her body in order for her to finish the. She did so, and Sanctus Nightmare returned to the physical world. He explained his plans on how he'll transform the world into an eternal nightmare, making everypony suffer with no end or break.
Twilight and her friends defeated him by destroying his left eye. As Sanctus Nightmare faded into dust upon his defeat, he promised his return, and how this was only the beginning.
Now, two months after their battle against Sanctus Nightmare, the Mane 6 head out to Canterlot for the opening of the Heroes Wing of the Canterlot Universal Museum. What awaits for them is nothing anypony would've expected.
