Dark magic is a more sisnister branch of magic, which comes in many verieties, while most forms of magic can be used negatitively these forms will always have negative cosequenses (though can be performed with good intentions).
True dark magic's (i.e. NEKTs') most infamous effect is that its' prolonged use leads to people losing or diminishing in some way their ability to feel certain emotions and 'virtues', including remorse, compassion, selflessness, love, empathy; all that good stuff. While a long-standing user can begin to notice signs of all of these feelings' degradation, usually they will start to diminish one by one, no particular order. No one understands for sure how or why this effect occurs, but it doesn't seem to occur even with extreemely prolonged use of other magics.
Depending on the classification of dark magic in question, the effect is swifter or slower.
There is superstition and theory among some about this topic. Some say that it erodes the soul from the evil that the user is doing. Others suggest the, again, soul is consumed by demons which are responsible for facilitating these forms of magic. Others believe that the cause is that dark magic is powered by emotion, and thus erodes them over time, though this doesn't explain why less positive emotions like anger remain. Some forms of light magic are also emotion-based, and they're not affected by this.
In truth, no one knows what causes the emotional degradation, the phenomena will likely remain a topic of heavy debate for the foreseeable future.
The second most infamous effect of dark magic, especially amongst teenagers is the addictive quality. Often brushed off as a mere myth by naive teens, the effect is no scare tactic. Witches who use dark magic will quickly find themselves compelled to use more and more.
Dark magic comes in four main types, which is why some (usually younger) withces refer to true dark arts as the NEKTs, as it's an aconym of the four main kinds. Other kinds of darker arts come within the umbrella of the following arts.