Background
For as long as the dark magics have existed, have there been those that have thirsted after their tempting, tainting power. Some, however, take it a step further, pouring their heart and soul into their study of the dark arts, literally... Be it a raw greed for power, the desire for revenge, or simply poor practice, the Shadowtouched are practitioners of the dark magics who have risked and endangered themselves in the pursuit of power and were changed by it...
Culture
Due to the rarer nature of the condition compared to other afflictions, and the individuality of each mage, there is little shared culture between sufferers. That is not to say that sufferers did not face a shared struggle in society. If there's a stigma against practitioners of the Necromantic arts, then sufferers are seen as the epitome of this fear and hatred, with some particularly zealous-few going so-far as to hunt those afflicted. It is for this reason that the shadowtouched often strive to hide the symptoms and nature of their condition. Of course, these issues are in no-way aided by the mental troubles which the affliction presents, with bouts of madness, hallucination, and even paranoia being especially common. This is only compounded further by a stark increase in dark desires, which the afflicted would never before indluge. In the more severe cases, these twisted wants and thoughts become so powerful, they may even manifest into separate personalities. Needless to say, they may grow to find it hard to trust others, and others to trust them...
Strengths and Weaknesses
Becoming Shadowtouched is by no means a pretty picture, and it most certainly is not a path that any reasonable person would intentionally set themselves upon. Those that suffer are wrought with a whole host of complications, which only grow in severity as their condition worsens. However, it also is not a path that one may easily escape, with the warping power being far too tempting to ignore.
The first such symptoms to appear are purely physical. Starting mild, the afflicted begin to develop painful physical mutations, regardless of their race, often leading to traits seen abnormal in humans, such as the nails growing long and sharp, like claws; slit pupils; or even pointed ears. As the affliction progresses, these traits only worsen. Common in all suffers, however, are black, purple, or even red spots upon the skin. As cases grow more severe, these spots become increasingly common, forming complex patterns and shapes as they cover more-and-more of the skin. Despite attempts to cover the patterns with makeup or similar methods, they still remain visible, leaving suffers to hide them under clothing, or reveal their condition openly. These discolored spots are also particularly vulnerable to divination magic.
Of course, sufferers find that its not just their form that is warped and twisted, but their very soul. As the Shadowtouched find themselves ever-more reliant upon, and entranced by their growing power, their soul slowly becomes warped and unstable. While imperceptible to most, the few that might see such, would find the sufferer's soul appearing cracked like a shattered window, meanwhile their aura often takes on a discordant mess of disagreeing hues and patterns. Its at this point that the sufferers' fate is "sealed," and their condition is no-longer curable, short of avoiding their magic entirely, which is far easier said than done.
Lastly, but certainly not least, are the various forms of madness which the afflicted are most often wrought with, ranging from paranoia and hallucinations; to strange, dark desires which are anathema to whom the afflicted are as a person. As these twisted passions grow more prevalent, the sufferer may often lose themselves as a new, darker personality takes hold. While an unstable soul may pose its own risk, biological 'death' is not a common symptom for most sufferers. Death of the self, as they became a shadow of their oldselves however, is a very real threat should the afflicted fail to contain against their urges. The only real 'strength' so to speak from this condition is an enhanced proficiency in Necromancy, and other such dark arts that the afflicted practices, even gaining a limited resistance against others practicing such magic. But even that comes at the cost of a stark vulnerability to the radiant energies present in divination magics.