YOU MAY NOT USE MY REDESIGN.
 
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So in my headcanon the Princesses are more Goddess then Demigods.
 
Luna rules over little compared to her «sister» [in this they are not actually sisters they just call one another «sister» as a mock form of equality betweent them]. She’s few centuries younger than Celestia.
 
Luna is very strong and also a powerful magic user, even more powerful than Celestia.  
Unlike her «sister» she doesn’t try to doll herself up to be appealing for all the masses. She knows her role and does it without hesitation and doesn’t feel she needs to be a public figure to do so.
 
 
Role: Queen.
 
Anatomy: The constellations around her tail and hooves constantly change shape depending on your view of them. Unlike her «sister» she isn’t to worried about if the public finds her to scary, she has no interest in making herself look ‘lady like’ and prefers a very fierce and powerful stance. Her job regularly doesn’t call for her to be ‘pretty’ and ‘modest’ in appearance. Scars she received from Timberwolfs, Dragons and Changelings.
 
Much like other pegasus Luna has a feathered tail. When flying she can make very sharp and quick turns and this makes her a dangerous fighter in the sky.
 
Job: Luna’s ‘Princess of the Moon’ title means very little to her, and she considers that her sister spreads her «ignorance» to visiting dignitaries. She tends to the security of the kingdom, dreams and more.
 
In the past Luna tended to the construction of her own jail, at the time it was needed, but with her fall from grace, the prison she built was destroyed by Nightmare Moon. Many dangers had been released. Now she’s in the process of a new prison, this one for less, world conquering villains.
 
Public interaction: She doesn’t care to much for the public or being in it outside of mandatory appearances, such as festivals. Though she is thought foul of because of her actions of Nightmare Moon, she is recovering some of her image by heeding her «sister’s» words.
 
 
Newborn  
Luna was born as a pure Alicorn. Luna was crowned “Heiress of the Moon and Planets” and duty to protect the young and helpless. She was venerated as Goddess of night, darkness and inspiration.
 
Filly  
Celestia and Luna always enjoyed playing with each other and simply spending time together. Her mother taught how to raise the Moon, but her parents disappeared for unknown reasons and filly was left on her own. Celestia, almost in her teen years, took the maternal role and strengthened her bond with the young Luna even more before the darker princess became more independent and started taking her royal duties more seriously.
 
Teenager  
Even though Luna put a lot of effort into her work as a ruler, Celestia always managed to get better results thanks to her wider and longer experience. This soon made Luna feel like she’d been set aside and almost forgotten - as soon as this idea crept its way into Luna’s mind, her slow but inevitable downfall began. She’d been working on a species-changing spell at the time, but hadn’t told Celestia anything about it. Proud and hatred-brooding at the same time, the Princess of the night was able to cast the completed spell on herself and transform into a more powerful creature. Luna magically took control of a part of the army, the night guards, under the name of Nightmare Moon. She began her rampage and attacked the castle ruthlessly, aiming only in the end at her «sister». Celestia tried in all ways not to hurt her, but the now stronger Alicorn left her no other options.
 
Nightmare Moon  
After her banishment to the Moon, Luna slowly and painfully reverted back to her original appearence, but the amount of magic she’d gained through the transformation didn’t lessen. The young mare’s cries and tears were ones of wrath and will for vengeance at first, but her emotions soon changed back to what they once were, when envy hadn’t taken control in her yet. She felt terrible guilt wash over her any passing minute and regretted her actions the more time she spent on the solitary satellite. When the millenium finally ended, the light magic of Celestia brought her back to the Earth and they reunited, feeling sorrow and happiness at the same time.
 
Young adult  
After taking a long time to heal past wounds with Celestia, Luna went back to her throne as second Queen of Equestria. The subjects paid her more attention and care since she returned as penitent, but in her dreams the dark Alicorn kept reminding herself of her rueful actions with the spell she’d once created. With the help of the Mane 6 she’s been able to overcome her sense of guilt, but this doesn’t mean she’s forgotten what she’s caused. Her role as ruler has got an increasing importance and she takes part in numerous councils and negotiations, even without Celestia.
 
Adult  
Luna is aromantic and asexual, she’s never felt romantic love for any creature. When younger, she had more than one partner, even two or more at once, but soon realized that she was forcing herself to be with them. On mutual agreement, the two «sisters» share their power equally.