Some facts about Princess Luna to help you write sad Luna stories:

-Betrayed her subjects and utterly failed in her duties ~1000 years prior to current events by giving in to her "worst instincts" and becoming Nightmare Moon
-As NMM, was one of the worst monsters ever to threaten Equestria, and the only one known that sought to kill her enemies
-Indirectly tried to kill everything on the planet by creating eternal night
-Destroyed the castle home she shared with her sister Celestia
-Nearly killed Celestia - who was only trying to flee - by shooting her out of the sky with a magic beam
-Was imprisoned for 1000 years on (in?) the moon
-Returned to Equestria completely unrepentant of her crimes, again inadvertently trying to kill everything
-Was defeated by Twilight and her friends when they were at their most inexperienced
-Lacked even the most fundamental understanding of how to interact with others following reversion from NMM
-Has no known close friends
-Aides ponies by entering their nightmares to help them understand and control their fears
-Has punished herself potentially every night since her reformation by giving herself nightmares
-She therefore spends virtually every moment of her life dwelling in nightmares
-Nearly destroyed Equestria a third time by unleashing the Tantabus, the instrument of her self-punishment, on the world
-Accepted the Tantabus back into herself after regaining control over it

Feel free to add more!

In my fanon, a city had grown up around the Castle of the Two Pony Sisters, and the fight between Celestia and Luna (which was entirely Luna's fault) badly damaged the city and killed a heck of a lot of Ponies.

Also, Celestia nearly went insane herself several times during her millennium alone.

Though at least my Luna's made a few friends since she's come back.

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Good point about Celestia. Imagine living with the knowledge that you had to imprison your own sister, and weren't able to prevent her from going mad.

Yeah, I prefer to imagine that NMM did more damage than she was shown to do. Particularly, I can't help but think she found some time, maybe while Celestia was unconscious, to eat some ponies (there's a grain of truth in every legend, they say).

In one of my fanfics Celestia reveals a missed out part of the Tale of the two Sisters, revealing that Nightmare had created a disease called Nightterror, which killed many members of the port Canterlot (which was not the capital at that time). In my view Nightmare also was not killed, but merley seperated from Luna and now tries to gain her host body back and to get revenge.

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Interesting. We do get the hint from the show that there's something still lurking in the castle ruins ...

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Yup. However I am not sure if that is Nightmare. Because in my opinion she keeps her dragon-like, cyan eyes. Even when she is defeated.

So a sad story would be Luna being tortured every day but still thinking she deserves it? Ooh, and what if someone she trusts did it? And she never dies! That would be a wonderful story.

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In my Fanon idea..... Princess Luna never regained her Full Magic but instead stayed as her Season 1 Design.

Crazy idea I know, but I just can't help but show some love for Princess Luna's S1 Design. It's still my favorite Design for Lulu :(

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And she was never able to regain full power or control...

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Hey! It's not that I don't hate Princess Luna's S2 onwards Design, I just think Fans like that Design of Princess Luna a little too much. Hence why I love Princess Luna's S1 Design a whole lot more than her S2 Onwards Design.

Long story short: I would take Princess Luna having a Normal Mane and Tail Style, aka her S1 Design, than her having a Flowing Mane and Tail like her older sister Princess Celestia, aka her S2 Onwards Design, any day :/

Oh, I like princess Luna as she is but what I'd really like is to get her old mane and tail back and then put it onto a taller Luna.

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Luan

Really -_-

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If it were done well, it would definitely be a contender for the Eclipsed Folder.

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Typo?

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yes. yes it was.