A thousand years in Equsetria's past everything seems bright for it's citizens but unbeknownst to them their rulers are still young and immature by Alicorn standards. The newer generations barely regard Luna as a major part of their leadership or their lives setting Luna on the path to become Nightmare Moon. But this is all changed when close to her breaking point Luna meets a creature who can grant her what she wants, to be loved while her elder sister is scorned. Her wish is granted but at a dire cost. What she thought was her dream come true became a nightmare that the princess of dreams could not escape.
Note: I made the possessed Celestia a little more darker than Nightmare Moon or the series's Daybreaker.