A Trim Reckoning: Darkness at River's End
Mundane volcanic lands sometimes feature black sand composed of ground-up cinders. Other than its striking color, such sand is no different from any other. However, magical black sand is a vile peril, whether on the scoured surface of Minethys in the Tarterian Depths of Carceri (where the Plane of Shadow overlays the Elemental Plane of Earth) or in lands cursed by foul magic.
Black sand is infused with shadowstuff and negative energy. A region of black sand literally swallows light; magical darkness rises to a height of 20 feet over the surface. Nothing short of powerful magics can disperse this darkness, and even so temporarily.
Maria informs you that a victim of black sand dissolves into more sand upon death. For skeletons to rise from it is… new. Concerning.