The following describes beliefs such as player characters trained in Religion might hold; it does not provide definitive information as to the reality of metaphysics in SyDarkSun.
SyDarkSun is different in several ways from the standard Dark Sun world. Primarily, it is quite a bit more secular. There is no conception of a ‘soul’ on Athas; either you’re alive or you’re dead. There is no notion of life after death, or of a spiritual essence that is distinct from your body. There is, of course, the life force that animates all living things, but if that is drawn out of your body, you simply die, and the energy can then be used to power defiling magic. There is no sense that your consciousness or identity is tied up with your life force.
Thus, there is no real concept of ‘undead’ as they are understood in other settings (including the RAW version of Athas). Necromancers may animate corpses, make them serve their will, and even imbue them with some tiny spark of intelligence, but these are not tortured spirits trapped on the material plane; they are simply dead hunks of meat or bone that have been magically manipulated. Similarly, powerful wizards may discover ways to continue living after their bodies would otherwise cease functioning, but—however ghastly they may appear—they are simply alive, kept so by magical intervention. However, both of these types of necromantic beings are considered ‘undead’ for game mechanic purposes such as a cleric’s Turn Undead feature. Such creatures are the result of unnatural necromantic interference in the natural cycle of life and death, and are often viewed as abominations, particularly by primal and elemental forces.
There is also no understanding of other planes in the accepted Athasian lore. Primal spirits and elemental forces are simply part of nature; they do not come from “somewhere else.” There is no knowledge—at least available to starting characters—of places such as the Shadowfell, the Elemental Chaos, or the Abyss. There is no ‘spirit realm’; primal spirits inhabit natural features of the land, and are probably mystical manifestations of the dwindling life force of Athas. Similarly, elementals do not come from the Elemental Chaos; they are the embodiments of the natural elements, and may be found residing in their elemental material—a water elemental living in a cistern or spring, or a fire elemental inside a volcano, for instance. There are rumors among the more esoteric loremasters of places such as the Grey and the Black, but such speculation is not known to most educated Athasians.
There is an important distinction, however, between elementals and primal spirits. Primal spirits—spirits of the land—are intelligent protectors of the remaining natural features of Athas. Elementals are, at least in the experience of most who encounter them, mindless or malicious manifestations of the elemental forces. However, there are differing views on the subject; someone damaging the protected area of a primal spirit may find it both mindless and malicious, and those who worship the elemental forces view elementals with reverence.
Possibly the closest thing to religion on Athas—aside from the power cults some sorcerer-kings engender—is the worship or reverence of the natural elements: Earth, Air, Fire and Water. There are no churches and no ecclesiastical hierarchy, but elemental priests are welcome almost everywhere, providing healing, guidance, wisdom, and some measure of faith. Devotees of the elements view them as the fundamental creative forces of the world, and feel that they have wills and desires for the bettering of Athas. More rational people view this as hogwash; the elements, while powerful, are simply natural forces, without will or purpose, and elementals—if not viewed as pure superstition—are simply another Athasian hazard. Nonetheless, elemental priests are clearly able to harness the power of the elements, and no one has a better explanation for how they do this than they do: That they are able to commune with the elements and are granted power in return for their devotion to the goals of the elemental forces.
There is no religious congregation of elemental worshipers; most people who feel a deep attraction or attachment to an element search out a priest of that element to learn its mysteries. Such people are rare; each priest can take on only one pupil at a time, and the process to become an elemental priest is long, arduous, and often unsuccessful. Only a chosen few are privileged to be able to actually commune with and control the elements. Most people practice whatever reverence they feel toward the elements as a matter of course, perhaps with small superstitious acts or by giving alms of money, food or clothing to the priests who visit their community. Elemental priests do not believe in charging for their services, instead depending on the generosity of others for their own material needs. This being Athas, that often provides little more than a bare existence. However, there are exceptions to this; besides the (not too uncommon) adventuring priests, there are communities who honor and care for priests who live among them, and some who even center their daily life around elemental worship. There are, for instance, dwarven villages that worship the Sun and revere fire clerics, and cults of Air are common among aaracokra. In addition, it is rumored that some evil priests are in the employ of the sorcerer-kings.
Of course, there are isolated cults and desert madmen that have views far outside of the norm described here, and the folklore and views of unsophisticated people can differ somewhat from the accepted viewpoints of those educated in religious lore.