Cassius Nightshade is DragonassasslnGR's OC that was modified from its original version for Dark Libra then reverted for Never Forgiven.
Life In The Series
The stories have Cassius placed as the Nocturnal Guardian of the balance of Harmony and Chaos and Princess Luna. He held the ultimate form of shadow magic that protected Equestria at night and made ponies fear the dark. He was lost to time along with his sister Sereen the Guardian of the day and Princess Celestia. The fall of the two was recorded only 2 year before Luna's banishment to the moon. After his revival due to forces unknown Cassius suffered memory lose until Luna reawakened him. Dooming all of Equestria to a war that nearly killed everypony. The final moments of what became later know as the Dark Libra incident. All seems lost until Mother Mare aka Lauren Faust's OC steps in and stops everything with the book of Creation that was almost lost to Cassius in his final moments in his psychotic episode to fix the balance by trying to kill Celestia.