Note: While this section contains no overt spoilers, it details concepts that are not necessarily revealed until later on. If you'd rather avoid even vague spoilers, do not read this page.
~The Soul's Home~
Within every "divine" soul (the souls of gods and humans), there is a small spiritual realm referred to as the "soul's home." The soul's home is a nebulous realm that reflects the person in question, generally molding itself to a form an environment they would find most comforting and personal. The soul's home typically contains a building holding objects representing what is most meaningful to a person—and most importantly, the soul's home contains records of one's memories, typically in the form of a library or well that they can gaze into. These are not limited to their current or most recent life, but rather contain all the memories of all the lives they have ever lived—such memories never genuinely reset. These memories are inaccessible to the soul's later lifetimes unless brought into their consciousness by a higher spririt (the gods and fae).
The soul's home is extremely difficult for humans to access, and made even more elusive by the fact few know of, or believe in its existence at all. However, one may theoretically be able to visit it with an absurdly high level of spiritual attunement; a level so high that it's all but impossible for one to be able to reach this level of inner reflection within one lifetime.
The soul's home of deities, however, works markedly different for the gods than for humans. For one, the soul's home of the gods both exists in the physical and spiritual worlds proper (except for Père Ciel's, due to his physical nature and overall detachment from the physical world). Theoretically, a human could physically go to a deity's soul's home, but for all practical purposes it's impossible (being in the middle of the Earth, Sun, and Moon). Another striking difference is that not only can gods easily access their soul's home, that tends to be where their soul lives outright—and not only that, but they can freely bring other's into their soul's homes, and visit the soul's homes others, god or human.
When a human soul dies, their soul detaches from their body, and they become a phantom. Phantoms exist in a liminal realm between the physical and the spiritual world proper—the physical world being where humans and animals live, and the spiritual world being where the higher spirits go when their spirit manifests outside of their physical body. Because they are technically on a different plane of existece, phantoms cannot see fae or gods any more than humans can; unless, like with living humans, these beings reveal themselves to them. Phantoms are considered the lower spirits, and in general tend to be very weak. Unlike the higher spirits, phantoms can only interact with the physical world in an extremely limited capacity without exerting a great deal of effort.
As a phanntom, one's soul steadily degrades; their memories recede into their soul's home, and their consciousness "resets" so that they might reincarnate properly. This process takes two forms: withering and corruption. For the first form, these phantoms slowly lose their sense of self naturally over time, in a way akin to a flower withering away. They harbor no particular resentment or malice, and eventually go on to reincarnate as usual. This process may take anywhere from a few months to a century to fully complete; however, most phantoms reincarnate before they've lost everything. In particular, they tend to reincarnate when they've lose most of their memories, but still recall a vague feeling of attachment to certain souls. This is a large cause for why certain groups of people have a tendency to reincarnate near each other over and over again.
The second form of degradation, corruption, is exactly as it sounds. It's very abrupt, and often happens immediately as the person in question dies. It's usually the result of violent death, and is more likely to occur to those that were cruel and hateful in life (however, depending on the severity of what happened, even more purehearted souls may succumb to this). These phantoms lose their sense of self incredibly quickly, and become fixated on a singular desire—usually revenge, or their desire to live again.
Corrupt phantoms can be extremely dangerous, and only become more so as time goes on. These phantoms are often violent, and may scratch, throw things at, or otherwise harm those who go near them. If the corrupted phantom has a grudge against one person in particular, they can summon enough power to murder them. These phantoms are also known to possess people—usually for short bursts, but if they have corrupted enough they could theoretically steal someone's entire body from them. However, their soul is so corrupted that they can't truly live in this state, and will probably soon die again of things like dehydration, exposure, and other causes stemming from a lack of ability to truly exist properly in the physical world.
Corrupted phantoms' danger is abated by the fact that they rarely can leave a small area, usually their place of death. Most tragically of all, corrupt phantoms can't reincarnate in their corrupted state. The only way a corrupted phantom can reincarnate is if they are purified by a priest or a god, cleansing them of their malice. Once this happens, they become a simple withered away phantom, and soon go on to reincarnate.
After one lives as a phantom for a period of time, they reincarnate to live again in the living world. How long it takes for one to reincarnate varies, but typically takes anywhere between a few months to a century; similar to the time span it take for a phantom to wither partially or fully. However, as previously mentioned, soul's make the choice on when they can reincarnate, and often do so before they have entirely withered.
Ideally, when a soul reincarnates, their consciousness will be entirely cleared and they can live (semi) fully as a new person. However, this process isn't infallible, and one can retaint remnants of the past life. This takes two forms; when one reincarnates before they have fully withered, and when one takes too long to reincarnate. The former case is actually quite common. In this form, one is born with (usually scattered) memories of their past life. These memories tend to fade away fairly quickly, usually being entirely gone by the time the person is five years old or younger. Because they fade so fast, and people are young when they have these memories at all, most born like this do not even remember these memories ever existed even by the time they are older children.
The latter case is an extremely rare occurance, when one remembers their past life because they waited too long to reincarnate. This happens when a soul doesn't degrade properly—because they spent too long "as the same person," it's like their present memories become seered into their consciousness in a way that cannot be simply erased. This process takes hundreds of years to solidify, far beyond the time span when most souls have fully degraded. This rarely happens naturally, and is usually a part of a concerted effort on the person's part to hold onto who they are even in the afterlife. Because this is such a difficult process, only a few hundred souls at the very most have ever been able to achieve this. When one remembers their past life in this manner, unlike with the first method, their memories never disappear even when they reincarnate again; for all practical purposes, they have become an immortal whose soul changes bodies occassionally. Like with corrupted phantoms, the only way they could reincarnate properly is to have their souls purified—but unlike all but the most corrupted phantoms, only the gods are powerful enough to purify a soul who has reached this state.
While the general fate for a human is to become a phantom and reincarnate, a very select few humans bypass this, and are instead whisked off to Heaven—or, more precisely, the soul's home of Lady Sun. Some time after Maiden Moon was imprisoned, Lady Sun revealed to her worshippers that she would bring them to Heaven as a way to promote order and peace in the world; and, too, as a way to boister her own standing to become the supreme goddess above the others.
Different people and religious sects have various ideas for how one gets into Heaven; some believe you must worship Lady Sun alone, others believe one must simply be a good enough person—yet, the truth of the matter is she simply chooses whoever she wishes for whatever reason, usually people she personally likes and wishes to keep around. Because of the, an extremely paltry number of people are allowed into Heaven each year, and they're predominately made up of nobility or clergy.