Millennia ago, before the three pony tribes came together, there existed a species of fae -- the succubi. Magic-eaters, they relied on ley-lines and the occasional generous unicorn to survive.
Eventually, however, their hunger grew to be too much. Without warning, they attacked the unicorns, draining entire villages and laying waste to the land.
The unicorns alone were not strong enough to fight them off. They begged for help from the pegasi and earth ponies, and together they cast the succubi into Tartarus and used them as gem-mining slaves.
At least, that's how the stories go. They have long since been forgotten.
Now, thousands of years later, a small group of sixteen succubi have emerged in the Everfree Forest, led by Pleione -- a soul shattered into seven sisters by a long-dead unicorn mage's spell, wanting only peace for her people.
After some negotiation with Princess Twilight, they have taken residence in the Castle of the Two Sisters, letting the alicorn study them and gather knowledge of their history in exchange for enough magic to survive.
But ponies delve deeper into the forest with each cycle of the moon, and the succubi who lived through the war are growing nervous despite Pleione's insistence that the war is over.
Lazuli and Azurite, twin sisters born in Tartarus, have no memory of the war, and no animosity toward ponies other than that born of the survivors' stories. At Twilight's request, Pleione has tasked them with reaching out to the residents of Ponyville, something they've taken to with varying amounts of success.
(Further info on succubi is here. I take no credit for the species, though I did create these characters.)
