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.
 
Nocte was a just a regular unicorn colt who was born and raised in the Crystal Empire, he didn’t knew much magic and was willing to be alongside the bests in magic. But one day, he adventured out in the arctic north and found a dark crystal, this dark crystal was the portal to the world of the Umbrums, dark spirits that are the reincarnations of ponies souls that lived in torment (basically they went to pony hell) Seeing how small and naive he was, the Umbrums tried to control him and possess him into a beast of shadows, and they did. Nocte became Sombra and he used his powers to cover the land in shadows, but he was stopped by Celestia and Luna who tried to contain him but he came on top. But he was sealed away in limbo alongside them in an effort to protect Equestria from him, however it didn’t last long as when Sunburst brought the sisters back, it brought him back. He continued his rampage while the sisters and the Mane Six pursue behind him, eventually they corner him in an ancient ruin and performed the spell to seal him away. However, a small glimpse of Nocte was shown, Twilight believes that he’s trying to escape Sombra but when she and Sunburst confront him, he tells them that it’s okay that they seal him in limbo because he already accepted his demise from the very day he became Sombra, so setting him free wouldn’t make a difference (basically, Sombra is willing to commit suicide to end his suffering) So they sealed Sombra and Nocte away in Limbo and make sure it can never to broken, he now lives in limbo for the rest of his life.