Everyone has their secrets…  
So! you all know my most recent fursona: StarDust.  
Well ive decided to add a bit of lore to her character, that being her Evil counterpart: BlackNight.  
She has a similar concept to Nightmare Moon, as in she is a completely different form to StarDust. She was formed in the back of StarDust’s head when her father: King Sombra, was banished along with the Crystal Empire.  
When StarDust become Luna’s student, she had multiple evil urges, similar to what her father went through with. That being enslaving ponies, taking over a whole kingdom and ruling over it for his own personal gain.  
So, princess Luna taught StarDust how to keep these urges and thoughts in check, in turn, keeping them stored away mentally, developing a split in StarDusts mind. Creating BlackNight.  
 
BlackNight, being StarDusts’s evil counterpart, is the embodiment of the pony StarDust avoids to be. But every full moon, no matter what, BlackNight will make an appearance, causing clouds to fill the sky, blocking out all peaceful and beautiful light created by the moon and Stars. She will also challenge Princess Luna to become the new princess of the moon, attempting to taker Luna’s place on the throne. Of course, she hasn’t been successful yet, so now, during her time being around, before sunrise, she will study in the royal library, looking for any spells that could block/freeze princesses magic. She’s managed to find said spell, but has never had an opening to use it. And so will, for now, remain an entity living in StarDusts head.