The Seventh Royal of Equestria. Aunt of Celestia and Luna, she was never meant to be Queen.
Althought at first she was only slightly jealous of Homonoia, this changed when Discord attacked her, as she was the bearer of the Seventh Element of Harmony, Hope(Who is now lost, as thanks to her corruption it became Cynicism).
When Discord got to her, she changed into a manipulative and cynic mare, and much to the surprise of her nieces, even pufyring her of the corruption wasn't enough, as she had taken a sick liking to creating strife among her people. Uncontrollable and unrepentant, The Bearers of the Elements didn't had other choice but to seal her away in a comet.
In modern times, she has returned, but, learning from her mistakes, and, unlike Discord and Nightmare Moon, she decided to keep a low profile, and used her magic to disguise herself as a Unicorn, but even her magic wasn't able to change her cutie mark(She was able to make it look simpler, though) and introduced herself to Ponyville as a traveler. Applejack mistook her for a family member(She had dubbed herself as "Ginger Gold") and has been manipulating the ponies to distrust each other.
When the time comes, she will reveal herself, and strike down her "dear" niece.
Ah yes. I lied, I made her an Alicorn, and the aunt of Celestia and Luna... which means she's the ancestor of Prince Blueblood. Eris, as many might know, is the name of the Greek goddes of strife and discord, while Homonoia is her opposite, the goddess of concord, unanimity, and oneness of mind(I was going to say Celestia and Luna's mother's name was Harmonia, but that was too obvious)
Anyway, original pony designs made by
My Little Pony belongs to Hasbro