Shadow magic is presented in Wizards of the Coast’s Tome of Magic. Necessary adaptations are outlined here. For more details, see the book.

Note: Very few know anything more about shadowcasters and Shadow magic, beyond its possible existence. Tales of shadowy magic are often discounted as the ramblings of drunkards and dullards, though some adventurers and sages speak of undead spellcasters who wield strange magic that stinks of darkness and death. More rarely, elusive mortals with warlock-like powers come briefly to light. This is the world of the shadowcaster, striding the gulf between living and dead, truth and myth, substance and shadow.

Shadow magic may date back to hoary Kelnore, though it appears to have almost died out several times in known history. Several cabals of shadowcasters are rumored to have operated in the Orichalan empire, and are widely believed to have gained some notoriety in the Demon Empires. In the modern world, shadowcasters operate openly in the Demon Empires, are an open secret in Viridistan and Tula, and hide themselves in the City State, Tarantis, and reportedly in Karak.

Shadow magic feels like arcane magic, and it is easy to mistake a shadowcaster for a sorcerer, wizard, or similar arcanist. Their powers create mana disturbances like normal spells, but they do not leave residual energy the way other spellcasting does. There is no ambient energy to re-absorb, and the residual signature fades at twice the normal rate.

Shadowcasters learn their fundamentals and mysteries more slowly than an arcanist learns spells, but these abilities gradually become an intrinsic part of the shadowcaster. While the caster’s most powerful effects will typically look like spells with somatic components, other effects at their disposal require less effort, functioning as spell-like or supernatural abilities.

Shadowcasters do not make the normal distinction between known and recallable spells. Rather, each mystery at their disposal is available per day once (if cast as a spell), twice (if a spell-like ability) or thrice (if a supernatural ability). Fundamentals are available thrice per day. After reaching the daily alotment, casting additional mysteries requires a Spellcraft check (+1 DC for each “extra” casting attempt of any mystery that day). Extra uses of fundamentals require a level check against the DC of the power, and do not get more difficult with repeated use.

The alien nature of Shadow takes its toll on all beings not native to the Shadow. Those witnessing mysteries for the first time must make a Sanity check (1d6 loss on failure, 1 on success). In encounters involving shadow magic, most parties lose 1 sanity point (unless they themselves wield Shadow magic). When a character gains their first level in a Shadow class, they suffer 1d10 Sanity loss (which may negate the 1d6 Sanity gain for leveling). Each time the character gains a sustaining shadow ability (or other major Shadow-related ability, at DM’s discretion), she suffers another 1d8 automatic Sanity loss. Those with the sustaining shadow ability can take a sabbatical to recover Sanity normally, but may do so either in the Material Plane or in the Shadowlands. Undead and certain outsiders are immune to the Sanity-rending properties of Shadow.

The sustaining shadow ability has special importance for undead: it allows them to bolster their essence with shadowstuff, reducing the threat and severity of withering. The duration between withering events (spell point regeneration, power point regeneration, attribute loss) is increased by one unit per sustaining shadow ability (e.g. a lich shadowcaster with three sustaining shadow abilities would regenerate one less spell point every four months, instead of every month, and it would lose an attribute point every four years spent without spell points, not every year). For this reason more than any other, shadow magic is popular among the undead — and therefore shadowcasters are viewed with suspicion among most mortal societies. Shadowcasting is also popular among the deceased, as it functions far better in the Shadowlands than conventional magic.

Shadow in many ways forms a bridge between the mortal world and the Otherworlds. As such, initiates of Shadow on both sides of the veil have some sense of goings-on on the other side. At first, those with levels in a Shadow class can simply sense the presence of one another in short range, whether in their present plane or in contiguous planes. As the character advances in her connection to Shadow, she gains greater sensitivity, particularly in sensing between the material plane and the Shadowlands. Upon gaining three levels in a Shadow class, she may communicate across the veil via gesture or speech, making herself understood to other shadowcasters, and with nine Shadow class levels this communication ability transcends language. With each sustaining shadow ability, a shadowcaster gains further influence across the veil between material and Shadow, choosing one fundamental, mystery, or spell that can cross planar boundaries. Any effects reaching from the Shadowlands into the Material Plane functions at half strength, and all damaging effects deal nonlethal damage to living beings. Under most circumstances, this ability does not work on consecrated ground.

The “bridge” quality of Shadow magic lends its living practitioners to serve as mediums and psychopomps, working with their deceased brethren to assuage the suffering of the dead and honor their memory. Shadowcasters on the other side find far more uses for their powers, and only a small fraction of them bother with the material plane. Those who do are known as ghosts, and may become deluctae (see Animae). Most communities in the Shadowlands have at least one shadowcaster who serves as a Lifespeaker, who passes messages and warnings to the living. Those that also have a living shadowcaster can engage in a much more direct dialog.