The standard D20 model of planar cosmology does not exactly apply to the world of the Red Tide. Planes such as the Shadowfell or Feywild do not exist in this world, and the Elemental Hells differ from the chaos of the canonical cosmology. Scholars and sages dispute the particulars endlessly, but most agree on the following general points.
First, the Elemental Hells exist as abstract locations embodying the principles of fire, earth, air, water, and potentially other qualities depending upon the theology or metaphysics of the instructor. These planes are commonly termed "Hells" due to their utterly inimical environment for any mortal creature, though their inhabitants are profoundly uncaring regarding mortals. Beings can be contacted from the terrestrial world with the proper rituals to call forth elementals and other primal energies. These powers are utterly disinterested in mortal affairs, and many philosopher-mages believe that an elemental or other primal power does not even possess sentience until a summoning ritual impresses a terrestrial form upon the power. Few dare disturb the Elemental Hells, as the rituals to call forth their powers are both demanding and prone to unfortunate consequences should they be miscast.
Aside from these realms is the pale radiance of the Astral Sea, a sort of formless gray sea of nothingness in which the Heavens and Hells float like bubbles, worlds of limited extent. It is said that great heroes of ancient days would go forth to voyage upon this spectral ocean, seeking the palaces of the gods and the smoking citadels of the Hell Kings. Even at the Ninefold Jade Empire's height, however, it was a rare arcanist who claimed to be able to pass into this place, and rarer still was one who could prove his ability to do so. The last known God-Seeking Hero vanished three hundred years before the coming of the Red Tide, and the secrets of how to enter the Astral Sea appear to have gone with her.
When a creature possessed of a soul dies, its shadow-self rapidly leaves the body and ascends into the Astral Sea. There, they drift outward in a dreamy, confused haze towards some unknown destination far into the gray waves. Most souls are irrevocably upon their course when they perish; no magic, however mighty, can call them back to the living world. A precious few have ties to this world powerful enough to allow them to return from the gray seas if the ritual of Raising the Dead can be performed quickly enough. The rite is both expensive and very demanding on the caster, but there yet remain as many a dozen dozen ritualists on Ektau capable of performing the ceremony, and copies of the ritual are known to be kept in Xian, Tien Lung, and Highgate. A scholar trained in the finer points of arcana or religion can cast a horoscope for any dead person which will indicate whether or not the rite would be effective for them, but hardly one person in a thousand proves amenable to the benefits of this rite, and it the venerable custom among astrologers to tell poor mourners that the rite would be useless for their departed. It is better that they are not tormented by the knowledge that only their poverty prevents the return of their loved one.
The temples which possess the ritual will not perform the ritual for those whom it would not help, despite the oft-tearful entreaties of the wealthy bereaved, and all performance of the rite must be done with the direct approval of the Mandarin. The ritual texts are kept under strict guard, and copying them without permission from the Mandarin is a capital offense, as is unlawful possession of a copy found elsewhere. Rumor has it that certain powerful scholars possess private copies of the ritual. The texts themselves do little good without adequate ceremonial expertise, however, and the Mandarin's spies are confident that there is no current danger to the Mandarin's monopoly on this powerful ritual.
Early on, these souls are relatively easy to contact, and Raise Dead rituals are effective. The more time that passes, the further into the void they spin, and the more powerful the magic necessary to reclaim them. Few souls remember anything of their time in the Astral Sea. Little more than echoes of nameless yearning and patient calm linger in their thoughts. For those devoted to a deity, however- or those unlucky enough to be wholly shunned by them- a different fate lies in store.
Each divinity has its own Heaven, a world forged according to that god's own whimsy. The faithful of a given deity believe that upon death they will be gathered up into their god's Heaven and will there receive the due reward of their pious service. The Nine Immortals promise gardens of fragrant peaches and springs of yellow wine for their chosen and their ancestors, and the Kueh believe that a righteous and honorable life will be rewarded with a place in the green fields and lush mountains of the land of ancestors. Even for gods of torment and the infernal Hell Kings, their faithful believe that they will be spared the agonies fit for lesser creatures, and their steadfast faith will ensure them high station in the everlasting slaughterhouses of their lord. Most spirits consigned to a Heaven are reluctant to respond to Raise Dead rituals and other modes of returning them to life, though it's possible for them to come back once more to the land of the living. The longer the time between death and resurrection, the more likely that they will be irrevocably wedded to the bliss- or torment- of their god's eternity. Those returned from the Heavens also remember almost nothing of the particulars, knowing only the vague recollection of completeness.
The Hells, on the other hand, are realms ruled by malevolent demons, or gods so close to such that there is no meaningful difference. They lie in wait within the gray void, seeking to snatch away souls that have no guardian deity to gather them in. The gods of compassionate and altruistic bent protect godless souls in sympathy with their causes, so that virtuous spirits are sheltered even without the explicit patronage of a specific deity. For the dead of more hateful or selfish bent, however, there are no gods to guard them. Without elaborate and careful funerary rites, their souls are vulnerable to being snatched up by the demons and dragged into the Hells, there to suffer unimaginable torments as they are slowly devoured to fuel the devil-god's power. Unsurprisingly, these spirits are almost always entirely willing to return to life, though the spell must be cast relatively swiftly if they are to be reclaimed before being hopelessly woven into the substance of the Hell. Those rescued from this awful fate are compassionately spared clear recollection of what they suffered, no more than scraps of nameless dread and a vague sensation of having escaped from some horrible place. The prophets of the Accursed claim that loyal service and worship of the Hell Kings in life will ensure an afterlife of rule over less faithful souls, and the satisfaction of every earthly desire in the red jade palaces of Hell.
Some scholars hypothesize that Undead are the shades of those wickedly godless or unfaithful souls who linger on this earth, too frightened of the waiting demons to take their journey into the gray void. Others dispute this on various empirical grounds, but it's a common peasant superstition that the wicked who lie unburied might come back as devil-ghosts. In consequence, it's not uncommon for peasant villagers to pay more careful heed to the funerary rites for a hanged bandit than the rites for an upright neighbor. Indeed, one of the usual funeral lauds for a dead man is a declaration that "He needs no rites from us," however questionable that claim might be.
Recently, the appearance of the True has led to the discovery of the Lie, a metaphysical construct that may or may not be an independent world. The Lie appears to consist of myriad possibilities and potentialities that failed to come about in the terrestrial world. The True appear to have some native power to manipulate the Lie, though other tainted souls are rapidly learning how to tap into the maddening and distorted "reality" of that ghostly consensus.
Amid all these metaphysical ponderings, no scholar has managed to present any satisfactory explanation of the Red Tide and the demons that boil within it. The tenor of the sorcerous energies involved does not appear to have anything to do with the Hell Kings, the elemental powers, or the very gods themselves. First-hand experience is unsurprisingly limited, but it was this very strangeness that helped make it such a scourge. Conventional defenses against devils and Hell-spawned powers were useless against the Red Tide- and indeed, the possessing devils of the Accursed seem to display an inordinate hatred towards the Red Tide and all its creations.