Two years after Nyx was stripped of the Nightmare's power, Twilight Sparkle is attempting to raise Nyx as a normal filly. But as Nyx's magical abilities begin to grow out of her control, the pair quickly earn the attention and ire of some powerful ponies who do not have Nyx's best interests in-mind.

During the night, the Elements of Harmony mysteriously vanish, and Princess Celestia enlists the aid of their bearers to find them. Twilight is forced to choose between her duties to her princess and her duties to her daughter. In the process, she discovers a web of conspiracy surrounding her daughter that reaches back a thousand years into Equestrian history. Will Twilight be prepared to face the things she learns along her journey's path: about Nyx, about Celestia, about Equestria, about the Elements...

...about herself?

Based on Past Sins by Pen Stroke Pony.
Alternate universe, because Twilight never became an alicorn or princess in this story.