Just throwing out some loose fluff concepts for the Shadow Power Source as a whole, no relation to any particular race, class, or other element. Some of my shadow source fluff ideas are tied up in Dragon submissions that still technically have a few days left before I can assume they've been rejected, but in the mean time, an idea re: the interaction between the Shadow power source and the Divine power source.
Shadow Heresies
"The so-called gods would deny the fundamental truth of our being, that our souls are born of darkness, and to darkness all souls eventually return. Darkness, you see, is the soul's true essence. It is only by rejecting their garish light, only by embracing the Shadow within us, that we may finally come to understand our true nature, and break the shackles of the gods' imprisonment."
~ attributed to the mad philosopher and arch heretic Selosus the Blind
The following is an excerpt from the academic journals of Voderick Pale, of the Nerathii Arcane Order.
Divine magic flows from the radiant light of the Astral Sea, the home of the gods and the seat of their power. Shadow magic, on the other hand, is drawn from the creeping darkness of the Shadowfell. Divine magic is that of ideals and perfect forms. It is magic of life and creation. In contrast, Shadow magic is that of blight and entropy, death and decay. Divine and Shadow are like day and night - polar opposites - and it is little wonder then that there has long been rivalry and conflict between the practitioners of these two arts. However, those who study the history of this conflict find that it goes much deeper then simple aesthetics. Indeed, it seems to stem primarily from that one element over which both the Divine and the Shadow hold sway: the mortal soul.
Through their servants the gods tell us that it was they who first brought life into being, they that instilled souls in mortal forms, creating their mortal progeny at the dawn of time. That connection is a deep and powerful one, and from it rises a deep and powerful emotion, a calling to worship. This calling can be diverted, but cannot be fully denied. Our souls are gifts from the gods, and rightly call out to their creators. It is only through devotion to the gods that our souls may know fulfillment, the most worthy servants of the gods carried into their domains to be made immortal. It seems only natural that Divine magic should have direct influence over our souls themselves. Or so the god's servants would have us believe.
Shadow magic also has influence over the soul, a fact that Divine practitioners like to bring up when arguing the dangers of using Shadow magic. Those same priests and clerics are silent when pressed on the metaphysical ramifications of this fact. Shadow magic does indeed influence the soul. Arguably, that influence is even stronger then divine magic, for Shadow magic's affect on the soul is much easier, much swifter. The spells involved require less energy. A shadow user can affect the souls of those around them incidentally, accidentally, which is never the case for divine magic. Many of these effects are negative or damaging, but not all of them. Shadow magic can sooth anger, drawing its energy into magical outlets, or instill lust. Shadow magic can alter ones destiny through manipulation of the soul. These non-damaging effects tell us that Shadow magic isn't purely antithetical to the soul.
There are other signs as well, particularly the eventual fate of mortal souls. With very few exceptions, mortal souls are drawn upon death to the Shadowfell, and then through it into the unknown. Why are souls shorn of their bodies drawn to darkness, rather then light, if indeed light was their true nature, the gods their true source? The gods are silent on the matter, and indeed worse then silent, for most of the major religions outright ban the study and investigation of the soul's eventual fate. What are they hiding?
There have been those throughout history who, so disturbed or compelled by these questions, have turned from the gods entirely to study the soul's connection to Shadow, and from these dark philosophers have risen a set of beliefs known as the 'Shadow Heresies'. Information on the Shadow Heresies is scant, for they have been actively suppressed by most divine cults throughout history. I have, however, managed to gather some scattered records, including a forbidden manuscript of the teachings of the infamous philosopher Selosus the Blind. His story is a compelling one, but I shall touch on that at a later time. For now, let me present some of what I've learned of these rarest of dark cults.
There are many different philosophies that fall within the category of "Shadow Heresies", but they all are united by a single core belief - that the Shadowfell itself is only the edge of a much deeper darkness beyond, a darkness which all souls seek to return to. They believe it is this darkness, and not the gods or their divine light, that is itself the original source of all mortal souls.
Selosus the Blind taught that the cosmos around us, not only the world and its echoes but the elemental chaos and astral see as well, all constitute a single 'world of light', and that there is opposed to this world of light a 'world of darkness'. Within the Shadowfell is a 'door to darkness', through which souls seek pass upon death.
Some of shadow heretics teach that the worlds of light and darkness exist in balance and harmony, souls existing in the world of light before passing to the world of darkness, where they are born to a shadow life. Upon the end of this shadow life the soul returns to the world of light. To those who believe in this cycle, the gods are shepherds and caretakers charged with maintaining this flow. These heretics frequently pay homage to the Raven Queen, though they might not outright worship her, and she seems indifferent to them.
Others, however, believe that the world of darkness is the soul's true home, and that the gods stole our souls from that home in order to draw power from our worship. Selosus was among these, and though he himself was not known for violence, he inspired several followers to strike out at the gods, their clergy, and their followers. Selosus believed that, even with Nerul gone, the door to darkness had not been unbarred. He believed that the Raven Queen and the other gods were diverting souls to fuel their own power, and that birth of new souls into the world of light was itself an act of kidnapping perpetrated by the gods. Selosus claimed to be in contact with beings from the World of Darkness, that the actions of the gods were damaging that world, and that nothing short of the annihilation of the 'world of light' could restore proper balance and return all souls to their rightful home.
Perhaps only the Elder Gods - those present at the dawn of time, know the truth, but if they do they aren't telling. Indeed, it seems that even the younger gods are unaware of the truth, for it has come to my attention that a number of Vecna's followers have been charged with pursuing the true nature of the soul and its fate, including investigating and even promoting Shadow cults and heresies - though perhaps the lich lord only does so to undermine the cults of his rival deities, I cannot begin to guess at the motives of such a being, and will speak no further on the matter.
It is worth noting that not every practitioner of Shadow Magic is themselves a believer in these Shadow Heresies - indeed these heresies have been so effectively suppressed by the churches of the gods that most Shadow practitioners have likely never even heard of them. But they do know that the forces of the Divine are frequently among their harshest persecutors, while Divine practitioners see shadow magic as a threat to the souls that their gods 'rightfully' claim as their own. Cooperation between Shadow and Divine magic users is thus quite rare, typically only seen in times of great crisis, or among small bands of misfit adventurers brought together by the fickle whims of fate.
Voderick the Pale was a prominent scholar of the Nerathii Arcane Order during the waning years of the Great Kingdom. Voderick was eventually cast from the order amid a flurry of accusations from high ranking clerics of the Churches of Pelor and Erathis - who variously claimed that Voderick was a heretic, a cultist of Vecna, a spy for foreign powers, and that he had poisoned a member of the royal family. Voderick disappeared before he could be brought to question, but a search of his laboratories revealed damning evidence of forbidden necromantic research. Voderick was never found, and most of his records were burned, although some of his published academic journals can still be found hidden within the remaining libraries of the Arcane Order.