PERSONA 5 IS CONFUSING AS HELL, so here's a guide to palaces, the meta nav, and stealing hearts
The Metaverse Navigator
Palaces
Mementos
Shadow Self
Treasures
Using the app Metaverse Navigator on their P. A. D., the protagonist and the Phantom Thieves of Hearts are able to discover the location of their targets and are able to infiltrate the other world known as the Palace to steal the hearts of "corrupt people." The protagonist attempts to delete it, but Igor restores it. He bestows the app to anyone the protagonist wishes to aid him in his rehabilitation. In order to infiltrate a Palace, three keywords are needed: a target's name, their location and the distortion the form of their Palace is based on.
Palaces
The Palaces are locations 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. 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. However, particularly corrupt individuals manifest personal Palaces that is solely inhabited by their Shadow Self.
All Palaces draw in Shadows, which help to protect them and fend off intruders and cognitive existences projected by the host. The stronger the person's corruption, the stronger the Shadows will be in the Palace. While cognitive existences do not affect their counterparts in reality, because the Shadows still represent the unconsciousness in reality, the Phantom Thieves of Hearts establish code names for all of their members to avoid identity exposure during their infiltration.
To activate access to a Palace, it requires to input the Metaverse Navigator app with four pieces of crucial information by voice: full name of the host, location of the Palace in reality, title of the host and the form the Palace takes. The Palace serves as the main battleground for the Phantom Thieves, as the main dungeons each represent one of the Seven Deadly Sins. Deep within a palace is a Treasure, a physical representation of the Palace owner's wicked or negative thoughts and desires. Once the Phantom Thieves of Hearts successfully infiltrate a Palace, find the treasure, and establish an escape route, they send a calling card to the owner in the real world in order for the Treasure to take on a physical form. Once a Treasure has been taken and the Shadow Self of the owner has been defeated, the Palace immediately crumbles and permanently disappears. The Treasures can be taken into the real world and can be pawned off. For the Phantom Thieves, the Treasures are used to fund their activities.
If the host's desires are destroyed by the intruder, that person's mental state will deteriorate in the real world and almost enter the vegetative state. If the host is killed outright, the one in reality will die as well. This is key to the plan of Goro Akechi, who uses the Metaverse as a way to safely assassinate his targets. The Phantom Thieves prevent mental deterioration by not killing the host but persuading the Shadow Selves to return to their true selves and confess their crimes. In cases like Yuuki Mishima's Shadow, a sound persuasion alone is enough to reform the host's mind without fighting or taking away the Treasure.
Mementos
Mementos is the Palace for everyone's hearts. It is shared as a common underlying palace that serves all of humanity.
Shadow Self
Shadow Selves exists for many people in Mementos, particularly for corrupt individuals. The vast majority of Shadow Selves merely take the form of a demon, except slightly stronger. Shadow Selves are generally more honest about their selfish motives than their real selves, yet cruel in their honesty when admitting to their corrupt actions. The majority fight against the Phantom Thieves of Hearts when confronted; however, Yuuki Mishima's Shadow Self was able to cleanse himself simply by being talked to. Futaba Sakura's Shadow Self, Shadow Futaba, was noticeably more kind and desired to have her confront her misinterpreted past. In the end, by accepting the truth that she herself and others had manipulated her into believing, Futaba was able to transform her Shadow Self into a Persona in a similar fashion to the Investigation Team in Persona 4.
Particularly strong, corrupted Shadow Selves have a dedicated Palace wherever their distorted thoughts are associated to their corruption. Unlike the more common Shadow Selves found in Mementos, Shadow Selves in Palaces have a stronger distortion of how they view themselves which affects not only how they appear, but also how their Palace and Shadow guards appear for them. In a sense, the Shadow Self is directly tied to their real world version. Due to this link, what happens to their Shadow Selves directly affects their real world selves. When confronted by the Phantom Thieves, they will attempt to fight and kill them to continue to hide their wrong doings. In defeat, they are goaded into admitting their guilt by the Phantom Thieves and within a few days or a few weeks after their Shadow Self is defeated, their real world self repents their former actions and usually turn themselves into the authorities to admit their guilt.
However, a Shadow Self can be killed instead, causing the real world version of themselves to experience what is described in-game as a mental shutdown. An assassinated Shadow Self causes the real world person to experience a heart attack-like affliction. The experience is noticeably painful for the individual as they gasp in intense pain while their eyes roll back and black liquid leaks from their eyes, nose, and mouth before finally collapsing on the spot, dead.
Treasures
The corrupted hearts all appear as treasures for the Phantom Thieves to steal, and when the treasures are successfully stolen the corrupted adults will feel remorse for their actions. Initially they appear as a formless cloud of rainbow energy, as the Palace's host is not focused on how they could lose it. However, if the host feels "threatened" of having a secret exposed, their treasure takes on a physical form, usually something related to the source of their ego. The Phantom Thieves achieve this by issuing a calling card directly to the palace's owner in the real world. Once taken out of the Palace, the host's world collapses into nothingness and the Treasure becomes a mundane but valuable item which the Thieves sell to finance themselves.
