This story is a sequel to The Fall of Princess Celestia
With Daybreaker's banishment about to be over in a few years, Luna needs somepony to be the hero to save her corrupted sister. Luna sees the potential of Twilight Sparkle and decides to make her student. Will Twilight Sparkle be loyal to Luna or will she become rebellious?
Background belongs to Magister39: https://www.deviantart.com/magister39/art/Canterlot-Throne-Room-550262042
Twilight Vector belongs to CloudyGlow: https://www.deviantart.com/cloudyglow/art/Twilight-Sparkle-is-a-little-angry-410444446