Before her banishment, Dimness Void had cast a curse on Canterlot, the city would never stop burning and its flares would last forever  
Celestia and Luna tried to extinguish the fire but all their efforts were futile  
Finally, they and the Royal Court were installed in The Crystal Empire  
Twilight’s betrayal broke Celestia’s heart and her magic began to weaken until one fateful day she couldn’t raise the sun again  
The chaos began to run among the ponies when the morning didn’t come, so Luna had to raise the sun as she lowered the moon  
Thereafter Princess Luna took care for both Sun and Moon, while she tried to enliven Celestia