"Kingsglaive: Final Fantasy XV" is a CGI animated film that serves as a companion piece to the video game "Final Fantasy XV." The story is set in the fictional world of Eos, where the kingdom of Lucis is under siege by the imperial forces of Niflheim. Lucis is protected by a magical barrier powered by the Crystal, a sacred artifact guarded by the royal family. The barrier is maintained by King Regis Lucis Caelum CXIII, who also commands an elite group of warriors known as the Kingsglaive. These soldiers, drawn from both Lucian citizens and refugees from Niflheim-occupied territories, wield magical abilities granted by the king.

The narrative begins with Niflheim proposing a peace treaty with Lucis, offering to end the war in exchange for the surrender of all territories outside the capital city of Insomnia. King Regis, aware of the empire's deceitful nature, reluctantly agrees to the terms to buy time for his son, Prince Noctis, to escape and fulfill a greater destiny. The Kingsglaive, led by the skilled warrior Nyx Ulric, is tasked with protecting the king and the capital during the treaty signing. However, the event is revealed to be a ruse, as Niflheim launches a surprise attack on Insomnia, deploying advanced technology and daemonic creatures.

Amidst the chaos, Nyx discovers that Niflheim's true goal is to seize the Crystal and eliminate the Lucian royal bloodline. The empire is aided by Ardyn Izunia, a cunning and enigmatic chancellor, and General Glauca, a traitorous Lucian commander with a personal vendetta against King Regis. Nyx, alongside fellow Kingsglaive member Libertus and Princess Lunafreya of Tenebrae, who is held captive by Niflheim, becomes embroiled in a desperate struggle to protect the kingdom. Lunafreya, betrothed to Prince Noctis, plays a pivotal role as she possesses the ability to communicate with the gods and awaken their power.

As the battle intensifies, Nyx makes a fateful decision to invoke the power of the Lucian kings by forming a pact with the Ring of the Lucii, a royal artifact that grants immense power at the cost of the user's life. Empowered by the ring, Nyx confronts General Glauca and the imperial forces, ultimately sacrificing himself to ensure the survival of Lunafreya and the future of Lucis. The film concludes with Insomnia in ruins, King Regis dead, and the Crystal in Niflheim's possession, setting the stage for the events of "Final Fantasy XV." The story explores themes of loyalty, sacrifice, and the cost of power, with Nyx emerging as a tragic hero whose actions shape the fate of the world.