I had basically a whole essay about her but I wanted to sum it up to the key points
AU - A more diverse equestria

Celestia
She/Her
Former queen of equestria
Talent breakfast chef
Children: Flurry heart

Summery: Founder of equestria, made the choice of banishing her enemies to the point where tartarus is filled by them. In an act to save her name she released her sister which caused a war to break out and her side losing. Instead of banishing her or killing her Luna stripped her of her alicorn hood and locked her away in the tower. She was allowed lovers though and children, which she has been plotting to use them to get back the thrown. One of these key player in her plot is Flurry heart daughter of her and chrysalis, which seems to be working. Her harassments pushes her toward her sister, who is aware of her linage but lets her pretend to be a dead foal, sooner or later she'll have her kill her sister but not right now.