To summarize what the AU is about, basically different events occur that cause certain events and characters to go and act differently than we know them. In this version of Equestria, it is ruled by the Pillars of Equestria (Starswirl, Mistmane, Rockhoof, Somnambula, Flash Magnus, And Mage Meadowbrook) who are cursed to be immortal.
 
Celestia and Luna were once rulers of Equestria, raising the sun and moon every day and night. However, they stepped up when Equestria was in mortal danger by the vicious Sombra, they tried many ways to contain and defeat him but none of them worked. Eventually, they formed a plan by using a spell to seal both Sombra and themselves in exchange of their raising spells being infused by the sun and moon so they can raise and lower at will. They perform the spell at Ponehenge and remained in limbo for 1,000 years until Sunburst brought them back as an attempt to bring back to two Alicorn sisters back to Equestria. Soon, they and the Mane Six team up to seal and contain Sombra once and for all.