Do. Not. Enter.

The Shadow is the dark reflection of the material plane, a dangerous place for mortal travelers indeed. Upon their death, all mortals pass to this plane. The proper observances summon divine minions to escort the deceased to the halls of their gods. Some rites also produce relics, shadow goods that the deceased may use for barter and protection. Without such rites, the dead soul awakens confused, naked, and alone in a stark and treacherous world lit only by the distant fires of hell-forges, the star-like sparkle of divine realms, and the silent face of Luna.

When a mortal soul crosses the planar membrane in death, it appears encased in a caul of membrane-material. This material dissolves soon after arrival, or may be cut away prematurely. Once the caul is gone, the character is officially Deceased.

Spell points and power points do not recharge here, and there is no ambient energy to absorb. Shadowcasters suffer no such impediment.

Adapted from the SRD (where it is called the Plane of Shadow):

The plane of Shadow is a dimly lit dimension that is both coterminous to and coexistent with the material plane. It overlaps the material plane much as the ethereal plane does, so a planar traveler with the right effects can use the plane of Shadow to cover great distances quickly.

The plane of Shadow is also coterminous to other planes. With the right spell, a character can use the plane of Shadow to visit other realities.

The plane of Shadow is magically morphic, and parts continually flow onto other planes. As a result, creating a precise map of the plane is next to impossible, despite the presence of landmarks.

The plane of Shadow has the following traits.

• Magically morphic. Certain spells modify the base material of the plane of Shadow. The utility and power of these spells within the plane of Shadow make them particularly useful for explorers and natives alike.
• Mildly neutral-aligned.
• Most of the plane observes no energy dominance, but some pockets of the Shadow may exhibit minor or major negative energy dominance. These may move like storms across the plane. Natives take half damage from these storms, and creatures with the Shadow template are immune to the effect.
• Impeded magic. Any magical attempts to leave the plane of Shadow are impeded. Also, spells that use or generate light or fire may fizzle when cast on the plane of Shadow. A spellcaster attempting a spell with the light or fire descriptor must succeed on a Spellcraft check (DC 20 + the level of the spell). Spells that produce light are less effective in general, because all light sources have their ranges halved on the Plane of Shadow.
• Enhanced magic. Spells with the shadow descriptor are enhanced on the plane of Shadow. Such spells are cast as though they were prepared with the Maximize Spell feat, though they don’t require the higher spell slots. All shadowcaster mysteries are maximized.

Furthermore, specific spells become more powerful on the Plane of Shadow. Shadow conjuration and shadow evocation spells are 30% as powerful as the conjurations and evocations they mimic (as opposed to 20%). Greater shadow conjuration and greater shadow evocation are 70% as powerful (not 60%), and a shades spell conjures at 90% of the power of the original (not 80%).

Despite the dark nature of the Plane of Shadow, spells that produce, use, or manipulate darkness are unaffected by the plane.

Excerpted from The World of the Wilderlands of High Adventure (where it is called the Shadowlands):

The Shadowlands is the transitive plane that touches on the Prime and all other Outer Planes. It looks much like the Prime, with much the same geography, but all washed out in gray, silver, red, and black. Plant life mirrors that of the Prime, but tends to be gnarled, withered, or sparser in general, and is often intelligent or at least blindly malignant. Cities in the Prime exist in the Shadowlands, but they are dead, rotting, ancient, and weathered versions of themselves; buildings that “die” in the Prime leave a warped, twisted version of themselves in the Shadowlands. These cities are haunted by lost souls and devils. The sun does not rise in the Shadowlands, though Luna does; there are many myths of the god or goddess representing Luna dying, descending to the Netherworld, and returning to the Celestial heavens thanks to the labors of a great hero. So day is night and night is as day in the Shadowlands. The silvery-red light that pervades the Shadowlands at all times is shed by the auroras that are found atop mountains near entries to the Celestial Realms, or from the fiery pits of the Shadowlands that lead down into the Netherworld. Heaven’s Bridge, the great planetary ring , is visible only on the occasions when it glitters gold (every 52 years), or on like occasions of celestial or infernal import.

When a mortal dies her soul makes its way to the Shadowlands, usually nude and bereft of goods, where she must await a messenger from her god, or perhaps her patron god himself, to collect her and take her on to her just rewards. Depending on the nature of the faith to which the soul held, this might require burial, prayer, and a ritual on the part of friends or allies still living in the Prime; thus the great importance of traveling with co-religionists! Souls of those who had no patron god, who are unburied or unhallowed, or whose lives were not notable enough for their patron to care for their soul are left to wander the Shadowlands alone, naked in the shadowy wastelands, where all manners of strange godlings, spirits, ghosts, devils, and demons live and feed upon or trade in lost souls. A few — a lucky, heroic few — are able to find a way back to the mortal realms of the Prime; of such adventures are great legends born and demigods made. Others may gain such spiritual power in the Shadowlands that they can make their presence known across the Planar Membrane; such ghosts and phantoms are most common in areas where the unhallowed dead can be found.

Several deities have domains in the Shadowlands, secondary homes, if not their primary, notably those who function as the psychopomp, or guide to the newly dead souls as well as those who judge the newly dead; those who have special interest in the undead or spirits; and those whose portfolio includes extraplanar travel. Notable gods and entities that may be found in the Shadowlands include Anubis, Arawn, Cerberus, Donn, Erebus, Erlik, Feninva, Fenris Wolf, Grismal, Harmakhis, Hecate, Heimdall, Hel, Hermes, Loki, Mawdorn, Mictlantecuhtli, Molna, Mondorent, Mungo, Nanna, Nephtlys, Odin, Orcus, Osiris, Phelonious, Set, Thanatos, Thoth, Xirchiriog, Yama, and Yog.

DUAT : This great cavern within the Shadowlands is the domain of Thoth and Anubis; here Anubis brings the souls of the newly dead worshipers of the Ghinorian gods. With his assistants Ma’at and Ammit, Thoth measures the weight of the sins in the hearts of their souls, noting the proceedings in his great Scroll of the Ages. If the weight of the heart is no greater than the weight of a feather, Ma’at, Anubis guides to soul to Aaru, the Ghinorian Heaven in the Celestial Realms; otherwise, it is taken by Ammit via caves dark and deep, to the Lake of Fire, there to writhe in pain and agony. The truly evil are consumed whole and destroyed by Ammit, as the Lake of Fire is not eternal, and souls have been known to escape, be freed, or even stolen therefrom.

THANOPOLIS: Thanatos has a city of the dead in the Shadowlands; here he brings all souls he finds that no other gods claim, or lose, and provides them shelter… at a cost. His sister/daughters, the Daughters of Night, go forth to battlefields to collect the souls of those who have fallen from their faith. Thanatos’ father/brother, Erebus, keeps a tower at the western end of the city, where he and his cohorts and hounds while away the days when they are not out hunting souls in the Shadowlands.

The Rememberers

The Rememberers are a vast and ancient tradition of deceased shadowcasters and outsiders who collect fragments of memory and dream from the Shadow (and, to a lesser extent, from the living world). This places them in competition with drifters, though the relationship is not necessarily adversarial. Rememberers tend to form into covens and societies, focusing on a particular geographic area, culture, or topic. The memories may reside in the Rememberers’ minds, or they may take the form of drifters’ memory pearls, an astral gem, a fey leaf/ butterfly wing fancy, or a shroud-like wisp dissipated from the deceased.

Rememberers occasionally employ astral beings to replicate important memories, usually to allow different covens to hold the same memory. This is difficult to arrange and quite expensive; the process is demanding and the astral artisans rare. Even worse, the astral beings consider the task onerous, and must be persuaded to perform.

Mortal shadowcasters (and others with connections to the Shadowlands) may attract the attention of Rememberers, for good or ill. Covens of Rememberers often keep stables of such contacts as a means of monitoring and acting in the world at large.