The Equestrian Dark Age

Alternate Equestria:

This is not the Equestria we know. Twilight never left Canterlot, never met the ponies of Ponyville or found the Elements of Harmony. Nightmare found them first, but before that, she killed Celestia and destroyed Canterlot. Her reign of destruction didn't end there. The darkness would not abate until all Equestria lay in ruin. Incidentally, comets are also within her power. In the chaos that ensued, Discord broke free. He saw that his playthings would die if the darkness persisted, but as he couldn't raise the sun or lower the moon, he made his own arrangements. Though the effect is less visible, the ever-present moon has emboldened the monsters the sea ponies keep under lock and key in the deepest abysses. They're free, and though they're never seen, the evidence is strewn across the shores.

Canterlot Before:

Canterlot pre-cataclysm is as it was before the Summer Sun Celebration, with one exception. This year, the celebration would be in Canterlot, hence why Twilight and Celestia were still present and awake when the stars aligned.

Canterlot After:

Canterlot post-cataclysm was a crucible where desperate ponies fought to escape the crumbling buildings, the firestorm, and the hail of comets.

The Castle of the Two Sisters:

The Castle of the Two Sisters is overgrown with moss and ivy, and the stones are so eroded that the moon is clearly visible through the ceiling, the forest through the walls and shattered windows. On the central pedestal, the Elements of Harmony rest.

The Hive:

Chrysalis's hive is a conglomeration of chitin spires that jut like spears out of the misty marshland her changelings call home. Strange flora and fauna inhabits the surrounding area, but the animals are only intelligent enough to know that the changelings are apex predators to be given a wide berth. Some are not so intelligent.

Manehattan:

Manehattan lies on the northeastern edge of Equestrian territory. It's a sprawling metropolis overlooking the ocean, ponykind's last stand against the encroaching darkness. The buildings and their walls are built to withstand the storms that come off the sea as if it were a watery Everfree Forest. The city has a significant gryphon population, and it's been the site of many failed invasions. The ponies there are hardened and cunning, but neither they nor their forefathers have ever known anything like the darkness.

The Crystal Kingdom:

A thousand years ago, the windigos encased the Crystal Kingdom in ice, Sombra's madness having let them in from the Frozen North. He'd fought them, then, but it was no use. The crystal heart was the only thing that could keep them at bay, and as his heart was no longer pure, it would not hear him.

The Surface of the Sun:

The surface of the sun is so intensely hot and bright, the gravity so powerful, that no one without magical protection of reality-warping magnitude can come near. Celestia's body was vaporized the instant it came in contact, but her consciousness persisted. Now the sun is her body, and wherever its light shines, she sees. The plasma surface churns and spits, and while such solar flares might not penetrate Discord's anti-mundane protections, there's no guarantee that they won't harm Twilight.

Hollow Shades:

Hollow Shades has a reputation as a haven for gypsies, vampires, and demon worshipers, which isn't far from the truth. The bat ponies who live there don't usually drink blood, but they are hunters, outcasts from pony society, and the descendants of Luna's night guards. When Luna became Nightmare, many remained steadfast in their loyalty.

The Ruins of Ponyville:

When the armies arrive, Ponyville has already been reduced to a smoking, fractured husk of its former self. Not everypony was able to evacuate in time, so crumpled skeletons litter the ground, and burnt flesh and burnt hair linger in the air. Craters are everywhere.

The Dragon Kingdom:

The Badlands are only the tip of the Dragon Kingdom, which borders the swamp where Chrysalis's changelings built their hive and extends far south of Equestria and far beneath the earth. The Ouroboros resides there, so massive he'd collapse under his own weight if he didn't have the tunnels around him for support. Only his eye is visible, and when its craggy lid is closed, the passage looks like a dead end.

The Ruins of Fillydelphia:

Fillydelphia, like Ponyville and Canterlot, has been reduced to ruins by the time the opposing forces meet to do battle. The only difference is the totality of the destruction. Hardly anything remains of the once-great city, and shipwrecks litter the shore. Even Nightmare is surprised; it wasn't her doing. Neither was it Discord's. It was the tide's.