Long night



Long night is an ecological disaster in the world of Eos in Final Fantasy XV brought about the release of light-absorbing miasma into the atmosphere. After sunlight diminishes from the world, Eos will be plunged into a perpetual night where daemons will take over. The long night is a culmination of the light and darkness theme of the game. The phenomenon is named in Final Fantasy XV: Comrades, and is also known as the "world of ruin" as per the name of a main quest in Chapter 14 in the main game, as an allusion to the World of Ruin from Final Fantasy VI.

Story
The coming of the long night is foretold in the prophecy of the True King, of a King of Light who brings light back to the world after it is veiled in darkness. It is implied the long night may have happened before, 2,000 years ago, when the civilization of that time collapsed.

Organisms infected by a mutant strain of Plasmodium malariae—Starscourge—disperse a light-absorbing miasma, which accumulates in the atmosphere and lengthens the nights. Daylight has gradually and sporadically been disappearing from the world for a long time, along with wildlife mutating and daemons proliferating, but it is not until the last Oracle dies that the effects accelerate to such a pace the general populace takes notice. After there is so little daylight left the daemons who thrive in the darkness can't be contained in one place, the Niflheim Empire collapses. Those who can take refuge in Lestallum, a city whose power plant is supplied by the meteorshards in the area. Angelgard is the last place in the world over which light shines, but once the sky above the island clouds over, the world is plunged into an eternal night.

When Prince Noctis returns to Eos after his ten-year slumber inside the Crystal, he finds a world of darkness where the sky is murky with dark particles floating in the air. Outposts have been abandoned, and trees and grass have withered. The waters around Galdin Quay that used to teem with fish are dead, and daemons roam the world. Noctis brings light back to the world by cleansing Ardyn Izunia's corrupted soul, the epicenter of the daemons and disease in the world. As miasma dissipates, sun rises upon Eos once more.

The state of the world after the sun returns is unknown, but it is presumed to have brought forth a mass extinction of wild life. Final Fantasy XV Official Works implies the world is rebuilt, as the tale of the True King bringing light back to the world is said to be passed down forevermore.

Trivia

 * In pre-release materials, the world was seen clouded in a red hue with wyverns circling the skies. This is likely an earlier version of the long night. Using an out of bounds glitch in Final Fantasy XV Episode Duscae also produced this effect, dubbed by many as the "". In the final game, the moon is not visible when the entire sky is obscured. The moon being red to herald a cataclysmic event is a recurring theme in Final Fantasy.
 * The long night hearkens to the end of the world events of the past series. Whereas in most Final Fantasy games the player party can avert the looming end of the world event, in certain games the player can witness the disaster first-hand, such as in Final Fantasy VI with the World of Ruin, Lightning Returns: Final Fantasy XIII with the Nova Chrysalia, and in Final Fantasy XV with the long night.