Dark Hour

"Would you believe me if I said that a day consists of more than 24 hours?"

- Shuji Ikutsuki to Makoto Yuki about the Dark Hour

The Dark Hour (影時間, Kage-jikan) is a supernatural phenomenon that occurs in Persona 3 as its main settings. The Dark Hour appears every night as a time anomaly that takes place during 12:00 AM, and serves as the 25th hour hidden between one day and the next.

Prologue
Ten years before the main event of Persona 5, where the occurrence of the Dark Hour started, the Kirijo Group led by Kouetsu Kirijo (who became obssessed with the Shadows after his long-term exposure to the Plume of Dusk) discovered the existence of the Shadows and Nyx Avatar. Upon realizing that they had the property to manipulate time and space, the Kirijo Group decided to perform experiments on them to refine and acquire their powers. Their project was carried out in the man-made Port Island, where a laboratory was built for such a purpose.

During the course of conducting the research, the Kirijo Group collected Shadows en masse and found out the prophecy of the Fall, an apocalyptic event to summon Nyx Avatar and bring forth the apocalypse. Accepting death as a deliverance and following the wishes of their employer, the senior scientists began rejuvenating Death, the appriser of Nyx to descend on humanity to bring about the advent of the Fall.

The Fall's Failure
During the final stages of Death's rejuvenation however, one of the senior researchers named Eiichiro Takeba opposed the efforts to initiate the the Fall, interrupting the experiment. As a result, Nyx was forcefully separated into thirteen distinct Shadows, each bearing their respective arcana, with the thirteenth shadow Death in an incomplete state.

The fragmented Shadows went berserk, destroying the lab and everyone within it. This in turn, made a significant impact on the time continuum, creating the Dark Hour. A tower named Tartarus soon emerged from the ruins of the lab, becoming the lair of lesser Shadows.

To cover up the incident, the Kirijo Group put all of the blame on Eiichiro Takeba, before altering all records of his findings and withholding all printed matter addressed to his family. This made him an essential scapegoat for the tragic incident. The ruined laboratory was later rebuilt as Gekkoukan High School, but every time the Dark Hour occurs, Tartarus reemerges, altering the structure to use as its own. When the Dark Hour ends for that day, the school transforms back into its original state.

Aspects
During the Dark Hour, the physical world is totally altered into a much more gruesome aspect, mostly with respect to atmosphere and color. The buildings, the night sky, as well as the moon would assume a disturbing green color, while bodies of water turn into blood, and puddles akin to blood pools form in random shall surface, even if there were not puddles there before. All electrical appliances cease to function, unless the equipment is specifically made for the Dark Hour by the Kirijo Group such as Mitsuru's motorcycle.

Normal humans transmogrify into coffins and become oblivious to what is happening. Shadows become active during the Dark Hour and attempt to lure humans out of their coffins in order to feed on their psyches, affecting them with the Apathy Syndrome. Those suffering from the Apathy Syndrome suffer from debilitating levels of apathy, leaving the inflicted collapse in a heap where ever they happen to be and become unable to move, feed, or care for themselves.

The number of victim cases grows in correlation with the approach of the full moon, when a shadow makes an appearance somewhere in the city. The cases quickly decrease each time a Shadow is defeated, only to slowly rise again as the next full moon approaches. People affected by the Apathy Syndrome are called the Lost by the inhabitants of Tatsumi Port Island.

The Dark Hour lasts for an hour and non-Persona users are impervious to it so the general public are unaware of the Dark Hour's existence. If an ordinary human survives the Dark Hour without being attacked by a shadow, their memories are erased instead.

Only those with the potential to summon a Persona are able to stay fully active during to Dark Hour and resist shadow attacks, and they are the only ones who can destroy the Shadows by using the gun-shaped Evokers and summon their Personas, formed from their inner self. Even if they sleep, the Persona users' bodies will not transmogrify, and will retain their memories. However, one without the potential can be taught how to enter the Dark Hour, as is the case of Shuji Ikutsuki. A known side effect of people able to experience the phenomena is a significant decrease in stamina: the longer they stay awake, the more quickly they will become tired.

Full Moon Operations
The Major Arcana Shadows, twelve powerful shadows that are fragments of Nyx, shall only appear when the Dark Hour occurs during a day of full moon, sometimes more than one. , wishing to return to its original body. SEES is tasked to kill all of them during these Full Moon Operations as per their chairman (Shuji Ikutsuki)'s orders, unknowing that he was using them to bring forth Nyx's arrival. In the meantime, they clashed with Stages, an apocalyptic cult that worshipped Nyx and tried to bring the Fall, taking advantage of Dark Hour phenomenon for their own personal benefits.

Trivia

 * The Dark Hour is compared to the TV World from Persona 4 and Metaverse from Persona 5, though it is both an event and a gateway towards an unearthly dimension, while the other two only serves as alternate realities.
 * It is not clarified what the range of the Dark Hour is and whether or not it appears outside of Japan as a worldwide phenomenon, or if it is exclusive to the timezone of Port Island, though it still appears at Yakushima. If it is a global phenomenon, then it can be assumed that it occurs at midnight for every timezone.