Shadow magic
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Shadowcasters
See the shadow magic of the Tome of Magic (3.5ed) for currently chosen system, but feel free to tip me about any alternatives not too dissimilar in flavor.
The arcane arts contain spells of the shadow subtype, but these figmants carry only a portion of the power utilized by true shadowcasters.
The magic they wield are of a different type from both arcane and divine in both source and expression, and will over time affect those who wield it in radical ways. Some have likened it to a consuming taint that eats away at a magic practitioner's gift until only it remains. Often referred to as "the creeping darkness".
As one might assume, shadow magic is tied to the Shadow Plane.