Abyss
Overview
Several years ago, the Anti-Shadow Weapon Metis sealed herself within an underground block of Tartarus known as the Abyss of Time, rendering it inaccessible. On the Dark Hour heralding the coming of the year 2010, the Abyss was revealed once more, leading to a month's worth of events that left their mark on the fabric of time-space itself. While this crisis has come and gone, the Abyss remains, drawing people to it still, its true purpose and origins yet unknown...
No one knows for certain what criteria may cause one to fall victim to the Abyss of Time. Invariably, however, those summoned hold some deep regret, some lingering doubt about the past that is often more deep-seated than they are willing to admit. Be it an accident they could have prevented or a choice they made with far-reaching consequences, the Abyss offers them the opportunity to revisit that past... for better and for worse.
The Summoning
A summoning to the Abyss takes the form of a door with a form familiar to the subject, appearing to them out of thin air, often in a time of distress. After crossing it, they are transported to the Desert of Doors beneath Tartarus. If they try to return the way they came through that very door, however, they will find themselves in their very own Memory Dungeon.
There is no obvious prize or punishment for going through one's Memory Dungeon. With it only comes the overwhelming feeling that the one called to it will find their answer. This, in itself, should be incentive enough: If the answer meant nothing to them, they would have not been called to a door in the first place.
A Memory Dungeon, much like a dungeon of Inaba's TV World, is a very personal experience, one built almost entirely from the subject's memories and perception of them. The rules may vary from one individual to another, and a Dungeon and its goals can take many forms, from a surreal, action-packed adventure to a quiet mystery to solve, or simply a gauntlet of battles peppered with flashbacks. Naturally, Shadows often dwell in these places, taking their form and power from the regrets and memories lingering within, sometimes leading to twisted, disturbing facsimiles of elements from one's past. For this reason, tackling such a dungeon alone is a foolish pursuit!
The Choice
If those summoned by the Abyss can reach the end, they are offered a unique opportunity: They get to relive the memory at the source of their regrets, and can change the event's outcome, be it by influencing their past self or taking on their own role directly. Should they choose to do so, the Abyss will offer them a glimpse of "what if": Immediately after enacting the change, all those present are gifted with memories from an alternate timeline in which this change has come to pass. To some, this knowledge may offer them the peace of mind they sought. To others, it may drive them even further into despair.
It's not impossible for an unpowered human to be called to the Abyss. Like much else about this place, the reason for this is a mystery, though the astute may notice that those drawn in share a minimum of supernatural awareness. Being trapped in it is a much greater peril to them, however: While the Desert of Doors offers safety from the Shadows in their dungeon, leaving to Tartarus proper will subject one to the physics of the Dark Hour once more, trapping them in the coffin they would normally become, to be drawn back in or (literally) dragged out. Without help, a human without a Shadow or Persona can effectively not leave.
For these people, however, accepting their regrets without changing the past will grant them a significant boon: the power of Persona. The artificial Awakening this induces is incomplete, however, and people granted the power this way are forced to use an Evoker to draw it out again. Changing the past and gaining the new memories will, on the other hand, not allow them to summon a Persona. This may mean, indeed, a painful choice between knowledge or power.
