Royal sister from the Acriverse! Zenith was previously named Solaria, but I thought the “time at which something is most powerful or successful” fits her character more.  
Here’s more info about them:  
After the three races of ponies came together, they chose a powerful, unnamed leader, that later birthed a lineage of previously unknown race - alicorns. Thousands of years later, two royal sisters were born with an unusual power of controlling the sun and the moon, a power that saved generations of gifted unicorns from working their asses off every morning and evening with sun/moon rising. Even though, the royal family was always loved, now the sisters were treated like actual gods and after their father’s passing, the oldest could finally rule like a real goodness.  
Zenith was always controlling but after becoming the Queen, she turned into a toxic control-freak. She needed to know every single aspect of every single pony’s life. She felt so important that she never listened to any consultants and just relied on her intuition. But! Her intuition wasn’t bad. She knew what to do to gain others’ trust and although she was manipulative as heck, many didn’t notice this side of her and just knew her as a strict but fair ruler. Zenith liked to look at herself through the eyes of her subjects, so it was hard even for her to notice that she slowly was becoming bitter jealous old lady. She started hating her own sister, convinced her that she wasn’t needed and one alicorn is enough to have control over both the sun and the moon. But Penumbra wasn’t your everyday pony that loves their beautiful Queen no matter what. She found enough power in herself to turn into…  
Equinox.  
A creature destined to bring equality between day and night. Many ponies secretely wished for her triumph but afraid of the Queen’s rage, they stayed quiet. Zenith saw those sparks of hope in others’ hearts and decided to put them down once and for all. She used the power of the Elements of Acrimony, often believed to be cursed, to banish her sister to the moon for a 1000 years. She did not only prove that Elements are to be adored but shown how fearless of a leader she is. The entire day was announced as The Day of the Queen, and was to be celebrated for a looong time.  
With the 1000th Day of the Queen at the corner, Zenith decided to kill two birds with one stone. She planned for Equinox’s return for a long time but she knew that she wasn’t as strong as she was before so she needed a group of ponies that could take her place as the wielders of the Elements. She send a student that she believed had enough hatred within her to evoke some kind of Elements’ power, not available before. And said student did, in fact, find a new way of using the Elements that didn’t harm anyone. Zenith saw the potential in gaining new worshipers and quickly took the opportunity to bring her sister to her side, mostly as an example of her good heart and power.  
Now, when both sisters rulle alongside each other (yeah, like Zenith would allow Penumbra to do something other than causing nightmares..), everypony seems to accept the fact that Equestria won’t change. As an almost immortal beings, the Queen and the Princess won’t be leaving their thrones empty soon, so it’s gonna be a loong time before the tyranny comes to an end. Oh did I mention that obsessive control turned into tyranny? It did, btw. And with every “villain” that decides to change their ways and join the rulers, hope weakens and weakens among those that wish for better days.
 
Some notes: