The Equestrian Dark Age

Twilight Sparkle:

Twilight is arguably the main character. She's there in the beginning, and she's there in the end. The same can't be said for anypony else. Though she fights like hell to survive and often has to wonder what the point is when things are so utterly bleak, she struggles not to lose herself. How easy it would be to give in. Her purpose in the story, more than anything, is to provide the eyes through which the reader sees the end.

Queen Chrysalis:

Chrysalis wasn't always queen of the changelings. She used to be an infiltrator, and a talented one at that. She learned much about the world on her assignments, and when she returned, she saw how the old queen had become corrupt and complacent. She usurped her and ushered in a new age for the changelings. No longer would they cower from ponies, barely scraping up what they needed to survive. They would take it, by force if necessary. That was her plan, a full-scale invasion, but then the Nightmare returned, and she knew she had to rethink her strategy, or there would be no hope for anyone, pony or changeling.

Nightmare Moon:

Nightmare Moon is the name given to Luna in the false history made up when Celestia, who had grown too fond of her subjects, chose to break the cycle of dark ages and golden ages. Celestia had never opposed her before―that wasn't her way―so Luna was taken by surprise. For the thousand years Luna spent on the dark side of the moon, staring into the abyss between the stars, the Ouroboros whispered to her, driving her mad.

King Sombra:

Sombra was a wise, benevolent ruler once. He was also Luna's consort. When she was banished, he turned to the dark arts in his grief. Shades consumed his soul in exchange for immortality, but for all his new-found power, he couldn't stop the windigos from taking his kingdom, for he had lost his connection to the crystal heart. It's doubtful that anything different would have happened had he learned of his lover's true nature, and what she was destined to do. The Ouroboros cares little for Sombra's contribution, yet he's more a pawn than anypony. Perhaps even a victim.

Princess Cadence:

Cadence is a minor character. More than anypony, she knows that love is the only thing that can carry her fellow ponies through the darkness. She also knows that her cutie mark depicts the crystal heart. With Shining along for physical protection as well as emotional support, she goes to find it and bring it back. She and Shining are also present to provide emotional torque for Twilight at several points in the story.

Shining Armor:

Shining is another minor character, Twilight's brother and last living relative. His presence is a comfort; his death will be ruinous. Notably, he loves Cadence. The feeling is mutual, of course, but it's not out in the open yet.

Discord:

Discord is an enigma. How he came to be is uncertain, but his history is rich and colorful. His actions certainly support his claims to the title of chaos incarnate, but he has been known to show compassion, often on a whim, usually when the end results amuse him. He stole the Elements of Harmony from the Tree of Harmony as a joke, and as an afterthought, before he got bored, imbued them with his soul, that he may never die so long as they existed. Doing so weakened him such that the princesses were able to use them against him, but they still couldn't kill him. When Nightmare shattered them, he was mortal again, but his stored power reverted back to him.

Princess Celestia:

Celestia knows what's coming, and she knows she's powerless to stop it. She and Luna are both mere reflections of the Ouroboros. She knows she's angered him―she's seen it in her nightmares―but until recently, she knew not how he would punish her. She'd hoped it would be just her, but it seems she wasn't so lucky. When she becomes one with the sun, her personality becomes so diffuse that she no longer recognizes herself as having any stake in the world. She's omniscient, omnipotent, and utterly indifferent.

Spike:

Spike was never meant to be hatched, merely to be kept safe, where the assassins that would seek to slay the last of the royal brood couldn't get to him. Ever since Twilight did hatch him, the two have been inseparable. He's a dragon among ponies, and she's a stranger among familiar faces. Apart from Celestia, she's the only family he has. Had he learned of his nature before being forced to join the migration, he still wouldn't have wanted to go. Since he was forced, he has less will to fight the Ouroboros's designs. The tributes that help him grow into the king his people need also help him forget the life he's lost.

The Ouroboros:

The Ouroboros is patient. The offense for which he seeks to punish ponykind occurred a thousand years ago, when Celestia's ponies urged her to defy the oncoming darkness. It knows neither malaise nor mercy. It seeks merely to perpetuate the cycles that define the course of nature and history. Self-preservation, in a way. It never interferes directly, for to release its grip around the planet would be to allow it to break apart, but as the incarnate god of the dragons―who are themselves cyclic creatures―it doesn't need to. Its body wraps around the world several times, and it communicates through telepathy.