Queen Celestia is considered the most powerful being in Equestria, even surpassing Discord despite his powerful chaos magic. She’s had one of the longest leaderships in all of Equestria, ruling for over five millenniums, most of which she has ruled alone seeing that Luna was too young to rule alongside her. Luna also spent a thousand years imprisoned in the moon, meaning Celestia had most of the royal power to herself for quite a while.
 
Luna’s transformation into Nightmare Moon happened rather early in the royal sister’s rule, happening only about a millennium and a half or so after she was old enough to rule alongside Celestia. During the time that Luna was banished to the moon, Celestia had to learn how to raise the moon, which was a chore all in itself in addition to raising the sun daily, but over time her magic got stronger and eventually it was easier to do. Celestia became a more strict ruler, and tended to stay out of the prying eyes of the public, she couldn’t stand being given praise for the worst thing she’d ever done in her life, she banished her sister to the moon, and everyone else was happy about it, but Celestia only felt guilt.
 
Before Luna had been banished, her and Celestia had to turn their close friend Starswirl to stone after he transformed into the creature Discord. This meant that after Luna’s banishment Celestia had no one to turn to, she was all alone.. Until she came upon a particularly talented filly: Twilight Sparkle.
 
Twilight became Celestia’s student after showcasing her raw talent for magic, and as she got older, Celestia couldn’t be prouder of her star pupil. Eventually Twilight was sent to Ponyville, where she was meant to make some friends, which she did after learning the fact that there was more to life than studying. Upon meeting these friends, Twilight discovered that each one of her new friends embodied an element of harmony, including her. Once the elements of harmony were discovered by Twilight and her friends, NIghtmare Moon was defeated and Celestia had her little sister back after all of those long years, meaning the sisters could rule as a pair again.
 
It took Luna some time to readjust to life outside of the moon again, and at first she was rather weak and had to be nursed back to health by Celestia. Once Luna was strong enough, she took her place alongside Celestia, and they had a small re-coronation as celebration for her return. Despite the sister’s royal subjects being happy for Luna’s return, many still feared her because of the tale of Nightmare Moon’s treachery, despite her transformation happening so many generations ago the tale was still told often due to the fact that it was considered a major historical event.
 
Celestia did everything in her power to dispel the rumors and shed some positive light on Princess Luna, and it worked for the most part. Celestia set up all sorts of events for Luna to attend, such as meetings with school ponies, meeting with the Canterlot elites, and making a few visits to Celestia’s school for gifted unicorns.  
Eventually things settled into a normal function, for a little while at least, until Queen Chrysalis herself came knocking upon Celestia’s door. The queen felt humiliated at the thought of asking for help but her people were dying, and Celestia was the last pony she could ask for help. Celestia couldn’t help the scowl on her face as she stared down upon Chrysalis from her throne, obviously angered over the changeling queen’s presence. Chrysalis proposed that they join kingdoms, making them a stronger group as a whole. Celestia couldn’t help but think this was a trick, but she went along with it, asking what was in it for her? Chrysalis guaranteed that her changelings would help protect Equestria, and that she wouldn’t allow any of them to harm Celestia’s subjects. Celestia thought about it, then agreed knowing that some extra protection could be useful, although there was one issue.
 
In Equestria, the easiest way to bind two kingdoms to one, is for a creature of royal status from each side to marry. Celestia was horrified at the idea of marrying a creature such as Chrysalis but she had no choice, it had to be done. The wedding was barely worth mentioning, for it was only done officially, there was no party or guests, nothing. The queens saw it fit that way, seeing that they were not marrying out of love but instead out of political advancements.  
After they were married for some time, Celestia decided that if she was going to be married to Chrysalis, she might as well get to know her, so she started attempting to spend time with the changeling queen. At first Chrysalis stayed distant and didn’t speak much, but eventually she secretly found herself falling for Celestia, gosh darn those pretty eyes!
 
Eventually they had a child, after Chrysalis laid an egg and infused it with Celestia’s magic, creating a child specific to both mothers instead of hatching as a normal changeling. They named their child Solaris, after Celestia’s late father. After the birth of their child, Celestia and Chrysalis were closer than ever, sharing strong protective motherly instincts, and protecting their little bug bab at all costs.  
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%~~Some other notes:  
~ Celestia loves to cook, hence why she makes Princess Luna breakfast every morning. There’s just something about cooking a homemade meal that relaxes her, she especially enjoys baking Hearth’s Warming cookies, because it’s family tradition for her and Luna to decorate them together.  
~ in her youth, Celestia and Queen Novo, or at the time: Princess Novo were very close friends. the two shared a close friendship, both just barely into their teen years when they met. Their friendship lasted until adulthood, until Novo and her whole family seemingly disappeared. Celestia was heartbroken when Novo stopped writing back so suddenly, and she feared for her friend’s life, but she had duties to attend to making her unable to travel to Mount Eris to find out what had become of her friend. Eventually after the Storm King was defeated, Celestia and Novo reconnected, and the two still write letters to one another often.