After the success of Luna's ascension, she was crowned as co-ruler of Equestria, serving the ponies alongside her former mentor. While Luna had hoped that she would be an equal to Celestia, this was not the case. Celestia could not move past the power imbalance of years past, with Celestia guiding and instructing Luna, who obeyed every word. Luna was an adult now, and wished for the freedom allowed by her majority and immortality. She dreamed of falling in love, ruling her people, and presiding over their dreams. Celestia did not share this dream, for her plans for Luna were much less grand and liberating. She expected her 'sister' to marry into a noble house, provide heirs to the ponies who followed her, and rule the world of dreams.
Luna despised this. She resented the stallions and mares she was betrothed to lifetime after lifetime, she was distant from the foals she bore in protest of their fragile mortality, and she grew tired of having authority in the dream world only.
She felt like she was a pet and servant to her sister, while the older mare ruled unquestioned. Celestia ruled Equestria with a firm hoof, earning and demanding respect and loyalty from her subjects, who all adored her and worshiped her as a goddess. Due to the apocalyptic nature of Luna's ascension, not a soul was told of the mare's origins, as many lifetimes had passed since her origin as the queen's student. Celestia kept the truth from her subjects, citing that the knowledge that the queen would have let them all die would result in hysteria and maybe even a revolt. The ponies were simply told that the queen's long lost sister had ascended to the throne and was the protector of the dream realm. Soon, the ponies began to wonder, 'what do we even need this second princess for? couldn't the queen do those duties as well?' and soon the Equestrian public opinion on Luna was tanked. While ponies would flock to see Celestia when she made appearances, only the most loyal followers of Luna would show themselves when she went out. Luna soon stopped going though with her arranged marriages, and Celestia stopped forcing her. The sisters grew apart, and after nearly a millennium of rule side-by-side, the two mares spoke maybe once a month.

not super proud of the little write-up for this one but i like the art i did. i wanted to show what the two of them got up to between the start of the rule of the two sisters and the emergence of nightmare moon.