For nearly one thousand years, the ponies of Equestria had a legend. A legend of a creature who resided in the abandoned Castle of the Two Sisters. An immortal being, one who had near omnipotent powers, but who only appeared in the Castle and its grounds at night, unable to leave. Ponies were wary of the creature, who was only known by the name Djinn, a Saddle Arabian name for a creature who could grant wishes to anypony who was brave enough to approach. This Djinn was unlike any creature known to the world, mostly because nopony who’d ever ventured to the Castle to ask for a wish never could remember what it looked like.
After a death in Ponyville of one mare who attempted to have her wish granted, Princess Celestia herself intervenes and visits her old castle to meet with the Djinn in pony, but is confused by what she finds...
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The concept behind this was pitched to me by yakopak.