The Dark Hour is an event that occurs each night during the events of Shin Megami Tensei: Persona 3. Most people are incapable of entering the Dark Hour and are unaware of its existence. For those that are capable of experiencing it, however, time and the world come to a standstill between midnight and 12:01AM. All living creatures incapable of entering the Dark Hour are frozen and appear as coffins.
Tartarus and the Shadows
As the Dark Hour begins each night, Gekkoukan High School transforms into the twisted, labyrinthine tower known as Tartarus. This is the only time period of the day that the player is able to enter and explore the structure. It is also only during the Dark Hour that Shadows make their presence known and prey on any unfortunate enough to be caught in the Dark Hour and unable to defend themselves. The SEES team the player joins at the beginning of Persona 3 is seemingly the only group aware of the Dark Hour.
Anyone present inside the high school when the Dark Hour begins will become trapped within a random floor of Tartarus. Should a person remain inside the tower when it transforms back into the high school, they will remain in the other "space" in which Tartarus exists. During the events of the story, Fuuka Yamagishi becomes trapped in such a way; when she disappears, the only clue to her whereabouts is her voice, which can still be heard in the school's halls as she continues to look for a way out of the tower.
The Dark Hour's End
As Persona 3 concludes, the Dark Hour is brought to an end with the defeat of Nyx. From that point on, those who had experienced the Dark Hour can no longer enter it, and Gekkoukan High School no longer transforms into Tartarus.
Persona 4 Arena
The Dark Hour is briefly revisited during Mitsuru's story path in Persona 4 Arena. When a government agent puts on a ring that had been included among a variety of relics and other evidence turned over to the police by Mitsuru, he becomes trapped in a localized Dark Hour.