What? I know I associated Final Fantasy XV with Fabula Nova Crystallis. And you thought they were separate, right? Well technically Final Fantasy XV’s mythology divorced from Final Fantasy XIII’s. But the common theme of Fabula Nova Crystallis is the idea that a god resides in the crystal. So it’s not clear if Final Fantasy XV severed from the Fabula Nova Crystallis series. According to Wikipedia:
Fabula Nova Crystallis Final Fantasy is a series of games within the Final Fantasy video game franchise. It was primarily developed by series creator and developer Square Enix, which also acted as publisher for all titles. While featuring various worlds and different characters, each Fabula Nova Crystallis game is ultimately based on and expands upon a common mythos focusing on important crystals tied to deities. The level of connection to the mythos varies between each title. The series title translates from Latin as ‘The New Tale of the Crystal’. Each development team was given the freedom to adapt the mythos to fit the context of a game’s story.
In Final Fantasy XV, the Astrals were gods, and Bahamut was such a god that resided in the Lucis crystal, so Wikipedia lists Final Fantasy XV as a part of the series. Now, of course some confusion might arise. According to the Final Fantasy Wikia page, Fabula Nova Crystallis:
is the collective name of a series of games made under the Final Fantasy XIII label by Square Enix. Made in the same vein as the Compilation of Final Fantasy VIIand the Ivalice Alliance collections, Fabula Nova Crystallis, which means “new tale of the crystal” in Latin, is based on various worlds and different characters, with each game “ultimately based on and expand upon a common mythos”.
And here, we can see no mention of Final Fantasy XV. However, the Wikia does end up acknowledging that
The connection between the Fabula Nova Crystallis games could be compared to the one that exists between games like Final Fantasy, Final Fantasy III, Final Fantasy IV and Final Fantasy V: different universes but roughly the same crystal mythos.
…leaving it up to interpretation that Final Fantasy XV could still connect to the Fabula Nova Crystallis.