This fic started out as a headcanon on tumblr-
"One theory is that the Astral's chosen one had their powers split into a king and oracle after the Ardyn fiasco. And when Luna died, she gave the Oracle powers to Noctis, the King of Kings. Imagine if Noctis could pass on his powers to Luna.
So, imagine a Final Fantasy game where you reach this moment, and the player gets to chose who lives and dies. Which one dies, and what are the consequences? We know how it is with Noctis, but what about Luna?
She has to deal with powers she never expected to have and training she never had to save the world. She has her own dose of guilt about Noctis, and she is constantly reminded by the presence of the guys who loved Noctis so much that they stuck with her - his future wife and inheritor of her powers - in her new found quest to save the world."

A huge thank-you to MilesAboveFantasy on Archive of Our Own (noct-noct-whothere on tumblr), for coming up with this headcanon and collaborating with me to plan out this fic! The stuff they've published is fantastic, so go check out their AO3 page :)
We're very excited to delve into Luna's character a bit more alongside the other chocobros, giving a little insight into the what-if of this very key moment in the game. Hope you enjoy the ride with us! Feel free to add us on tumblr (noct-noct-whothere and finalfantasyx).

SPOILER WARNING! THIS FIC WILL CONTAIN SPOILERS. IF YOU HAVE NOT FINISHED THE GAME, YOU HAVE BEEN WARNED.

Disclaimer: I don't own Final Fantasy XV

In the days of old Lucis, there was one in line to take the throne. So generous, so selfless was he that when the world was on the verge of being plunged into eternal darkness in the war of the gods, the Great War of Old, he was the first to respond to the call of the gods, offering himself for the sake of the world at the beckoning of the Crystal that was given to him by five of The Six. The Scourge came, and with the help of the First Oracle, he opened his body to host the darkness, restoring the light to Eos, but at a heavy cost.

The Crystal and the Kings of the Lucii, their lights tied to the Ring given to the King of Lucis, denied him ascension when his time came; it took his given name of Lucis Caelum and gave it to another, one who was called to take the Crown. The Savior's heart was no longer pure, steeped in darkness, and for that, Providence cast him out. He was banished from Lucis, and slowly, his demonized powers grew to the point where he became immortal. Betrayed by the one whom the Crystal had chosen, betrayed by five of the Astrals, he pledged himself to Ifrit, the Bringer of the Night, shrouded himself in the darkness he hosted and disappeared, history blotting out his name.

It was because of him that the Astrals divided the power to push back the darkness, now known as the Starscourge- half to the Kings from the line of Lucis, half to the Oracles from the line of Fleuret, chosen by them. It is because of this reason that the King and Oracle should stand together always, for it is they who safeguard the world.