Her story is known, her defeat & destruction, and Princess Luna's happy ending...
Though, It in fact was not the end, just the beginning for Nightmare. As those that don't perish, often grow. Whether it's for good or ill is dependent upon the circumstances. These particular circumstances resulted in her becoming a complete pony of her own. Ever thankful of the new friend that gifted her that which she was missing, she persevered.
Into the future that she had built, the ponies that she brought into the world, the new family she made, and eventually the 'empire' (collection of colonies really) that her ponies forged amongst the stars. Far far from Equus.

However, with such growth comes longing. Ponies have an inherent desire to see their families, old and new. Now that she is one, the desire to really know them and love them has long awakened.

'Since Mother and Auntie are alicorns, they must still be alive out there...somewhere?'

With so much that she needs to apologize for, so much to say, so much to know... the day she can see them again just cannot come soon enough. However, it's been more than 9 centuries now, that they thought she was only a thing, and destroyed at that.
Her wait is at an end, though there are always bumps in the road of life.

Though this story is set upon Earth, there are only ponies there now. The world 'building'(unwritten prequel) is a slightly apocalyptic one. Which could even be considered a bit of Nightmare's penance; to have to helplessly witness the end of a civilization (and side step being swallowed up by), in order for her to get a second chance & a clean slate to make her own future. Not everything was lost of them however. After all, where would the idea's of advanced technologies have come from otherwise?
Prequel here: The Early Days

This story takes place many centuries after, in better days.
Part 1; is the lead up to the reunion, and world building. The other princesses do not yet appear (tags adjusted). Also re-rated teen due to married couples (slight comedic romance)