O'er rotted Soil, under blighted sky, A dread Plague the Wicked hath wrought. In the Light of the Gods, Sword-Sworn at his Side 'Gainst the Dark the King's Battle is fought. From the Heavens high, to the Blessed below, Shines the Beam of a Peace long besought. "Long live thy Line, and this Stone divine, For the Night when All comes to Naught." 15:2 "Nadir"
Starscourge is a plague upon the world of Eos in Final Fantasy XV that lengthens the night. More daemons have been appearing recently, which people attribute to the longer nights. Only the Oracle, the current one being Lunafreya Nox Fleuret, can keep the plague at bay and stop the world from plunging into perpetual darkness where daemons will take over. Starscourge is a cloud of photosynthetic organisms that drink light before it reaches the ground. It is the ultimate source of daemons as commonly occurring parasitic protozoa. Those afflicted with advanced stages of Starscourge exude black mist from their bodies.
In the Old Calendar Era, the astral god Ifrit betrayed the rest of his kind and caused the downfall of the nation of Solheim. Ifrit began the Great War of Old that killed millions. Ifrit sent forth the Starscourge, photophilic organisms that parasitize people and transform them into monsters.
Ifrit became known as the Betrayer, and is opposed by the other astrals who want to purge the darkness Ifrit brings forth. The darkness of the Starscourge is countered by the powers of light. Bahamut, who appears to be the leader of the astrals, entrusted the Crystal to mankind 2000 years ago. It exudes great magical power, and is said to hold the soul of the world. Bahamut put his plan to purge Starscourge into motion by handpicking two people, one whose bloodline would become the Crystal's protectors, and the other the first Oracle.
The Oracle has the power to counteract the Starscourge and hold it at bay, and their bloodline became the royalty of Tenebrae. The bloodline that protects the Crystal became the Lucian kings, who are granted power to combat daemons and be empowered by the Crystal. The astrals' last gift before they fell into a deep sleep was the Ring of Lucii, which would house the souls of the Lucian kings. The Starscourge subsided by the king's hand, and the Final Fantasy XV Scenario Ultimania posits the power of the royal arms, combined with the power of the Crystal, can destroy the Starscourge. Legends tell of the king of Lucis forging covenants with the astrals two millennia ago, and fighting alongside them against the darkness. The scourge did not disappear from the world entirely however, and it was feared it would once again overwhelm Eos one day.
The Lucian royalty can combine their powers across generations via the bonding of the souls, as the one who wears the Ring of the Lucii can wield the powers of the kings of old. Thus, the power of the Crystal gradually concentrates until the king who wields all of the Crystal's power will be crowned the True King who can purge the Starscourge at the cost of his own life. This became the prophecy: "When darkness veils the world, the King of Light shall come." With the plan to cleanse the world of Starscourge in motion, Bahamut disappeared from Eos, and the tales of the astrals became legends.
It appears the Oracle's healing people of the Starscourge is to also buy time for the Lucian lineage to accumulate enough power for the True King to appear. He is said to be even more powerful than the gods and to have the power to banish the darkness from the world. The Lucian kings are sworn to protect the Crystal until that day arrives, and gain the ability to use various magical powers as facilitated by the Crystal. The Final Fantasy XV Scenario Ultimania describes the Crystal as having immense magical power, but that the kings of Lucis don't wield its full potential; rather, they have the responsibility of protecting it. The Crystal picks the "Chosen King" when disaster comes to the world.
Millennia ago Ardyn Lucis Caelum was a member of the Lucian kings' bloodline who was chosen by the Crystal to save Eos from the Starscourge. He absorbed countless daemons into his own body, which saved thousands across the world, but resulted in him being rejected by the Crystal and denounced by the astrals as unclean. He was denied entry to the afterlife and demonized by a jealous future king who later became chosen by the Crystal in his stead. As his transition rendered him immortal, Ardyn became the embodiment of daemonic influence in the world and the Starscourge. He took the surname of "Izunia" and vowed to one day have his revenge on the Caelum dynasty and the Crystal. Whether Ardyn was, in fact, the first king of Lucis, is unknown, but Starscourge is said to have been abated by a Chosen King two thousand years ago, and Ardyn himself claims the Crystal had chosen another as his replacement. Thus Ardyn was cast aside and forgotten.
Ardyn bided his time for millennia before establishing himself as chancellor to Iedolas Aldercapt, the Emperor of Niflheim. He provided his knowledge to begin his plan for revenge. With Ardyn's coaxing the Empire began to use daemons for experiments for creating a magitek infantry, and to deploy as weapons of war. A mysterious "vanishing disease" began to ravage the imperial lands. Although the Starscourge is not a new phenomenon, it was poorly understood, and for a time the vanishing phenomenon that has people disappear into thin air was not understood to be a disease. In truth, the people who become afflicted with Starscourge disappear when they turn into daemons.
After Ardyn murders Lunafreya, the effects of the Starscourge accelerate. With no heirs, the Oracles' bloodline is extinguished when both Lunafreya and Ravus Nox Fleuret are killed. Ardyn lures Prince Noctis, the last of the Lucian lineage, to Gralea, the capital of Niflheim, where the Crystal is now kept. The Crystal had always kept the Crown City of Insomnia safe from daemons, and Noctis believes he can save the world if he can salvage the Crystal. As Noctis touches the Crystal with the Ring of the Lucii however, he is pulled inside it. In an otherworldly dimension that exists on the inside of the Crystal, Noctis meets Bahamut who explains to him that he is the True King.
After Noctis disappears, the sun never rises again. Over the following decade, the Starscourge overwhelms Eos, covering the world in an eternal night and plaguing it with daemons. After Noctis awakens ten years later, he returns to the ruins of Insomnia with his friends to stop Ardyn and end the Starscourge. Ardyn pits Noctis against Ifrit, but Noctis defeats him with the help from the other astrals before encountering Ardyn in the Citadel's throne room.
Overpowered by Noctis, Ardyn is finally able to rest in peace. His body fades away, and his spirit is destroyed when Noctis—who gave his life to halt the Starscourge and purge the daemons—uses the power of those he cherishes and the Lucian kings of old to land a final blow. As Ardyn's soul is erased, the darkness is lifted, daemons disappear and the sun rises. The prophecy fulfilled, the Ring of the Lucii disintegrates as the Lucian bloodline ends with Noctis's death.
The advancement of Starscourge is shown in the game by its day and night cycle. In the beginning the party wakes up from a rest at 6 AM. By Chapter 8 the party wakes up at 9 AM and the sun already starts to set after 2 PM. In Chapter 12 the party will get up at 10 AM, and Chapter 13 onwards, the player won't see sun again. After Chapter 13, the Starscourge will completely envelop the world and the clock and day and night cycle are removed. In Chapter 14 Eos's atmosphere is a grisly blackened sky lit by an obscured sun that sheds black dust particles onto the world, rather than the starry and moonlit sky portrayed in the natural night phase.
Daemons spawn on the field after nightfall due to Starscourge. They are powerful monsters that will aggressively target the party. Players may stay up to battle daemons, though they can shave off maximum HP. Iron Giants and Red Giants have high levels and resistance, unless they are daemons fought under a Hunt. The game advises for players to make camp or to rest at outposts and towns due to their danger they pose to an underprepared party, and as obstacles that hamper the progression of travel at night.
If the player has finished all the sidequests from Cindy, they will get the Enhanced Headlights, which will prevent daemons from spawning in the road when they travel at night.
Starscourge taking over and spelling the end of the world could be seen as allusion to various end times described in several world religions. It could also represent a manifested concept of "original sin". To bring "original sin" to absolution and repentance requires one to turn to Jesus Christ as their savior, who came to die for the sins of mankind in the New Testament. In Final Fantasy XV, Noctis can banish the darkness from the world at the cost of his own life.
Kegare is a prime concept in Shinto, Japan's national religion, describing of the importance of cleanliness and the need to wash away impurity, which is believed to stem from disease, filth, and the result of death and bodily harm. Such states of impurity were believed to spawn demons and evil kami. One old belief of Japanese superstition was that sickness was the result of demons. With the Starscourge said to be comprised of that of photosynthetic microorganisms, and the original name for daemon being that of "corpses," (シガイ, Shigai?), the Starscourge works with the context of kegare as a malevolent disease that reanimates the corpses of the dead into unholy creatures of darkness.
- Dualism of light and dark has been a theme in the Final Fantasy series since its inception, the crystals and their Warriors of Light representing light. Other symbols to the theme includes the world being named Eos (a goddess of dawn), Luna's and Noctis's names referring to being lights in darkness, and the gods of the world being sometimes referred to as "the stars".
- Starscourge has some resemblance to Chaos from the Fabula Nova Crystallis: Final Fantasy mythology as a corrupting force on the world. Final Fantasy XV was originally part of the subseries as Final Fantasy Versus XIII.