The Palaces

"One could say it's a world in which one's distorted desires have materialized. I call such a place a "Palace.""

- Morgana

The Palaces (パレス, Paresu) are a set of the main dungeon setting in Persona 5. They reside in a dimension called Metaverse. A Palace is a manifestation of "distortion", strong negative and corrupt thoughts that warp the perceptions of people into a hazard for themselves and others (for example, in Junya Kaneshiro's Palace, people were shown to be living ATMs).

According to Morgana, most people's negative and corrupt thoughts are blended together into the Metaverse in a location called Mementos, which is a Palace for all of humanity. Only the extremely distorted people would have Palaces for themselves.

Each of the Palaces represents one of the Seven Deadly Sins.

Mementos - Prison of Regression
Mementos, also known as the Prison of Sloth or the Prison of Regression, is a Palace for the general public's distroted minds. Mementos is divided into multiple "paths", each of which has its own unique architecture and floors, referred as "areas". They are randomly generated and areas open up as the fame of the Phantom Thieves grows with each Palace they defeat.
 * Available: May 7 (December 24 for the depths)
 * Deadline: Few days after December 24 (Rearrest)
 * Owner: The people of Tokyo / Yaldabaoth
 * Location: Shibuya Railway
 * Treasure: Holy Grail (Yaldabaoth's base form)
 * Path of Qimranut: Consists of 2 areas.
 * Path of Aiyatsbus: Consists of 7 areas. (rest on area 4)
 * Path of Chemdah: Consists of 8 areas. (rest on area 5)
 * Path of Kaitul: Consists of 10 areas. (rest on area 6)
 * Path of Akzeriyyuth: Consists of 12 areas. (rest on areas 4 and 8)
 * Path of Adyeshach: Consists of 13 areas. (rest on areas 5 and 9)
 * Path of Sheriruth: Consists of 14 areas. (rest on areas 6 and 10)
 * Path of Iweleth: Consists of 1 area and acts as an entrance to the depths of Mementos, within which the Holy Grail has hidden itself.

Like all other Palaces, Mementos symbolizes one of the Seven Deadly Sins. In its case, its sin motif is Sloth or Apathy. Theologically, Sloth is the most complex of the sins, encompassing laziness (the desire to perform evil deeds or let them exist unopposed because it is easier than doing good), cowardice (evil born of fear), and despair (disaffection from life motivating selfishness and abandonment of moral and ethical duty).

Acedia (Apathy) is the neglect to take care of something that one should do. It is translated to apathetic listlessness; depression without joy. It is related to melancholy: acedia describes the behavior and melancholy suggests the emotion producing it. In early Christian thought, the lack of joy was regarded as a willful refusal to enjoy the goodness of God; by contrast, apathy was considered a refusal to help others in time of need.

When Yaldabaoth awakes, and the Conspiracy starts invoking its power to control Tokyo directly, it is shown in the city's populace choosing to believe an obvious lie (that somehow, the Phantom Thieves are not responsible for the high-profile reforming of several individuals), rather than a cutting truth (that Shido was a terrible leader, and yet most of Japan still voted for him because of nothing but raw charisma), showing cowardice.

As the Phantom Thieves manage to find their way into Mementos' core, it's also revealed that the Shadow Selves that had inhabited Mementos have been willingly imprisoned by Yaldabaoth, choosing to let him dictate the future rather than allowing fallible humans to make their own decisions, showing both the laziness and despair aspects.

After the Holy Grail expels the party from Mementos, the real world begins fusing with it. The party is transported into the Velvet Room, where the true identity of the Grail is unmasked. Upon escape, they find themselves in the Qliphoth World, and begin the ascent to take down the Grail.

After the destruction of Yadalbaoth, the evil God of Control then returned back to its former form - a souless Holy Grail in a much smaller size. After that, with the core of Mementos' distortion destroyed, the entire Mementos - along with the Metaverse as a whole - was erased.

Palaces

 * The Palace is somewhat similar to the TV World of Persona 4, as each dungeon typically reflects the mentality of the one who created it.
 * Kamoshida's Palace
 * Kamoshida's Palace is similar to Yukiko's Castle in Persona 4. Kamoshida, who views himself as the 'king of the school,' which is reflected by his Palace, with the appearance of an old castle, and Yukiko Amagi, whose dungeon was a castle that represented her feelings of being locked away by her own birth.
 * Sae's Palace:
 * Sae is the only major target (excluding Futaba who does not have a villainous role and neither does her Shadow) that truly redeems herself and becomes an ally to the Thieves, without having her Treasure stolen or her palace destroyed.
 * Sae's Treasure is the only one that is never explicitly shown, either in the Palace or out of it. Prior to its materialization, however, Makoto theorizes that Sae's real world Treasure is their father's notebook on investigations he's done, as his death in the line of duty is the major catalyst of her cynicism.
 * Shido's Palace:
 * E5c4QWf.jpg Mementos as a whole doesn't count, Shido's Palace is the largest Palace in Persona 5 with the longest running time in the main gameplay.
 * A neon sign on the side of the ship has the words "The arc of elite."
 * The submerged city symbolizes both the progressively rising sea level because of climate change and the economic state of Japan which is still struggling to recover from the burst of asset price bubble in 1992.
 * Originally, Shido's Palace was even grander than what we saw in the game. It had a giant ferris wheel, a roller coaster that went into the sky, a space shuttle launch pad, complete with rocket, and behind all of that was an entire city located at the back. It also took place at night, with Zeppelins patrolling the sky. All of those above were eventually scrapped in the final game.
 * Shido's Palace is the second cruise dungeon in the Persona series after Nichirinmaru from Persona 2: Eternal Punishment which also happens to be the headquarter of the conspiracy.

Mementos

 * Sandro_Botticelli_-_La_Carte_de_l'Enfer.jpg overall design of Mementos is reminiscent of La Mappa Dell'Inferno painted by Botticelli and based off of Dante Alighieri's written work The Divine Comedy.
 * Mementos is an auto-generated dungeon like Tartarus and the Midnight Channel.
 * Mementos becomes mandatory in December, as it generates a new palace to be entered by the Phantom Thieves.
 * In English version, the paths of Mementos are named after the Qliphoth, specifically the versions given by William G. Gray in his book The Tree of Evil. This ties into the ultimate master of Mementos being Yaldabaoth, as the Qliphoth is taken to be the "shadow of God", just as Yaldabaoth is only a deluded impostor of Ain Soph, Kabbalah's name for God as He was before the creation of the world.

Misc.

 * It was revealed that the Traitor had his own Palace, but it was eventually scrapped, and the Phantom Theives confronted the Traitor in Shido's Palace instead.