UPDATED:  

 
Backstory:
 
Selenite is a bat pony mare born on the 21st of December 980, her childhood home was a furnished cave in a small community of bat ponies not far from Ponyville. At school, she excelled in all subjects but showed a noticeable interest in history and politics, despite those two subjects not being taught that much in her education. Nonetheless, she researched these two topics in her own time, learning a great deal about military doctrine, government functionality, and Equestrian history, taking a keen interest in the banishment of Nightmare Moon, which she believed would soon come to an end.
 
On the night of Nightmare Moon’s return, she was a mere 19 years old, upon hearing that the return of the lunar princess came exactly when she thought it had, she celebrated with her bat pony friends as they believed that this would be their kind’s “liberation”. But when the sun came up and Nightmare Moon was no more, she was fairly disappointed. Nonetheless, Luna’s return prompted her to enrol in the Equestrian Night Guard’s Officer Corp, where she showed a level of competence never seen before in Equestrian military history. She quickly rose to the role of Commander in Chief for all Equestrian military units and has become a close friend of Princess Luna herself.
 
BONUS! Nightmare Takeover Timeline alternate backstory:
 
Selenite is a bat pony mare born on the 21st of December 980, her childhood home was a furnished cave in a small community of bat ponies not far from Ponyville. At school, she excelled in all subjects but showed a noticeable interest in history and politics, despite those two subjects not being taught that much in her education. Nonetheless, she researched these two topics in her own time, learning a great deal about military doctrine, government functionality, and Equestrian history, taking a keen interest in the banishment of Nightmare Moon, which she believed would soon come to an end.
 
On the night of Nightmare Moon’s return, she was a mere 19 years old, upon hearing that the return of the lunar princess came exactly when she thought it had, she celebrated with her bat pony friends as they believed that this would be their kind’s “liberation”. Nightmare Moon’s return prompted her to enrol in the Night’s Army’s Officer Corp, where she showed a level of competence never seen in Equestrian military history, making short work of any resistance to the new order. She quickly rose to the role of Commander in Chief of the Night’s Army and has become a close friend of Nightmare Moon herself. Surprisingly, she has used her position to influence Nightmare Moon to embrace a less totalitarian perspective on civil rights and such.
 
EDIT: Removed the shading so it’s easier to interpret.