In this AU, Tempest Shadow has the role that Luna does.  
 
Princess Shadow and Princess Shimmer were the princesses of the night and day respectively. They protected equestria together and ruled side by side until Princess Shadow had been injured and her horn broken.  
 
Given that she was an alicorn, her horn could be healed and regrown in time. However, her sister would be the sole rular until then. To speed up the healing process, she found a gem of great power called “the storm crystal” but that’s when the real danger began.  
 
She watched as her sister took over her duties and was praised and loved by all the subjects while she was left in the shadows with the crystal by her side. Soon, the crystal began to speak to her. Began to prey on her insecurites.  
 
“You see? Your sister wants to rule by herself.”  
 
“You could be far more powerful and even more beloved if you merged with me.”  
 
“I can make you whole again.”  
 
Princess Shadow let the crystal consume her and transformed into a being known as “the Storm Queen”. She tried to destroy her own sister and claim the throne as her’s alone. In order to stop her destruction, Shimmer sealed her own sister away in stone. This would last for 1000 years as Shimmer continued to search for a successor to hopefully heal her sister when the time would come that the spell would wear off.  
 
Thankfully, Starlight and her new friends managed to stop the Storm Queen’s return and fully free Princess Shadow of the evil influence, allowing the two sisters to rule side-by-side again.