No one gets through life without regretting something. Many people often think, 'if only I could change x, my life right now would be so much better'. But the past is the past, and it can't be changed.
Except that the nature of the Dark Hour and the Tartarus can cause some oddities, to put it lightly, where 'normal physics' are concerned.
Six people vanished into the Abyss of Time beneath the Tartarus towards the end of 2009, and upon exploring what lay within it, those six people were given the option to change what they most regretted. Some accepted, some didn't. The root of the distortion was determined to be the fault of the Greater Shadow The High Priestess, one of those Shadows released in the Kirijo Incident that created the Dark Hour in 1999. Though she was defeated, and though the power of the Abyss faded, it didn't vanish entirely. Something otherworldly still lives beneath the Tartarus, and it still seeks out those who bear a great regret upon their shoulders.
The Nature of the Abyss
The Abyss sits in the very bowels of Tartarus itself (see Dark Hour), accessible normally through the passageway leading downwards in the lobby of the twisted tower. For those who take the long staircase down into the depths, they find the Desert of Doors, a sand-filled location with a dizzying number of doors of various shapes, sizes and colors. These lead to Memory Dungeons, violations of space and time, that ultimately drag the character and anyone with them over into a temporary possible future based on the alteration of one crucial moment in the character's life, when something happened to them that they very much regret.
With the appearance of a new Memory Dungeon, a new door appears; they can't be seen or found by anyone until that event. Furthermore, opening the door to a Memory Dungeon can only be performed by the 'owner' of that dungeon--this applies for any subsequent visit after the initial threat of time and space alteration has been dealt with.
The Desert of Doors is a 'safe' location, free of the variety of threats that stem from the Dark Hour's peculiar physics. There are no Shadows in the Desert of Doors proper, for example. Electronics also function, and there is no Transmogrification effect for civilians. Each Memory Dungeon, however, may ultimately waive certain of these rules, with one sole exception: all dungeons will possess wandering Shadows of some type.
The appearance of a Memory Dungeon is marked by the appearance of a familiar door before a character, wherever they stand anywhere in the world. This door is always connected in some way with some sort of action that they deeply regret. For example, some whose moment of regret involved a ballet rehearsal might be visited by that very same door that lead into the building or room where the rehearsal was taking place. The door /can/ be seen by other Persona-users and Shadow-Possessed, but not by any civilians (save of course if it is a civilian character getting sucked into the door in the first place).
The compulsion for the character to open the door is overwhelming. Once the character does so, they vanish from sight in a flash of light, the door disappearing along with them. They find themselves in the outer reaches of their dungeon, typically with the exit close at hand. This is a fortunate thing, for a Memory Dungeon is fraught with danger and should not be attempted alone. Upon their escape, they find themselves in the Desert of Doors, and upon their exit from there will find themselves within the next Dark Hour, regardless of how much actual time they /think/ has passed.
Though they have escaped for now, the presence of their unexplored Memory Dungeon weighs heavily on their mind, no doubt stirring up afresh the very regret that brought the dungeon into this world in the first place. They must explore it and set their mind at ease, but it is extremely important that they don't go alone. What dangers might lie ahead within the dungeon are never a certain thing, and sometimes regrets really can be killer.
The Memory Dungeon
Every Memory Dungeon is different. Many things are possible within a Memory Dungeon. The dungeon of one character might consist of flashbacks to key memories broken up by fights with powerful Shadows. Another character might have a devious series of puzzles that need to be solved before they can progress into the dungeon. Frequently, the surroundings take the form of twisted recollections of important places or themes in the character's life. For example, the ballerina might have contorted and trap-laden theatre, where the hallways don't ever act as hallways ought to.
Shadows are encountered within Memory Dungeons, consisting of the wandering variety typically found within the Dark Hour.
Whatever the road is like, the character and their allies will eventually reach the center, where plays out the memory of the character's greatest regret. The time and space distortion nestled in the heart of the dungeon awakens, dragging all present into its grip.
The Alternate World
Within the distortion, a version of an alternate world plays itself out, all contingent on the alteration of that single moment. The character who spawned the Memory Dungeon is cast into the role of their alternate self--and similarly so is everyone with them and their alternate selves. Whether the character is completely subsumed into their alternate self, changes and all, or are merely playing the part is up to the individual players. However, acting wildly different from however they are supposed to be in this alternate world will cause the NPC denizens of this world to notice that something's wrong. They may not take kindly to it.
The exact length of time spent within this alternate world depends on what the character and their allies have planned, but typically shouldn't run longer than one OOC week.
There are a number of ways to escape this alternate world. Other characters may opt to choose to hurt the person who spawned the world. Another option is to simply reason with the person. All alternate worlds include a doorway back into reality, but it isn't easy for anyone to find. Finally, there is always some object anchoring the alternate reality that if destroyed will return everyone to reality.
All of these will typically prompt a manifestation of the regret that brought this all into being in the first place, whether after they attack the character, convince the character, find the door out, or are just on the verge of breaking the anchor of that world. They are as varied as what spawned them and invariably aggressive. Perhaps the ballerina from above would cause a monstrous being symbolizing her perceived failure to appear within the world. Whatever its form, they must be killed to escape.
Whatever happens to banish the alternate world, all characters who played a role within it remember what this world was like, their role in it, and their changed relationships with the characters who were there with them. Furthermore, any injuries or deaths that occur while in the alternate world will remain.
Civilians and the Abyss
Characters who have not awoken to a Persona or are not Shadow-Possessed may develop a Memory Dungeon. While the completion of a Memory Dungeon and the alternate reality it spawns does not spur them on to develop a Persona as a matter of course, they may, as a result of what they have experienced as a result, gain a Persona or become possessed by their Shadow. Particularly, the Persona Game and the Evoker method are of relevance here: the threat of the Abyss may be a sufficient trigger for a character who has played the Persona Game, or it may mark the emergence of the Potential for a character, meaning that they could become an Evoker user. Or perhaps the trials and revelations were too much, and the regret too crushing. A character may become Shadow-Possessed from the experience (see Shadow Possession).
Civilians face an additional hurdle. While the Desert of Doors is 'safe' and lacking in the presence of Shadows, the moment they step up into the lobby of the Tartarus they become subject to the full power of the Dark Hour once again. This includes Transmogrification, should they be subject to that effect of the Dark Hour (see Dark Hour)
*As implied by the above text, a Memory Dungeon is never a solo affair. Much like with a TV Dungeon, a character should gather friends and allies before attempting to brave what lies beyond their door.
*Any unusual uses of the Abyss that don't cleave to this general layout above will require a TP application.