A Manual of Maladies: Slate Skin

Slate Skin is an affliction that causes the dermis of the patient to become hard and brittle. It is believed to have had its origin in the Underdark and only escaped to the surface among the demihumans who are particularly susceptible to it. Slate Fever propagates through contact with others infected with the sickness. Early symptoms include a slight mottled discoloration of skin tone, stiffness of the joints and a loss of sensation on the surface of the skin.

The symptoms eventually progress to the point where the diseased becomes noticeably grey and loses a high degree of locomotion. This causes them to move in slow, short steps and to slough off hardened bits of skin when forced to do more. 

The disease itself usually passes by itself over the course of one week spent recuperating in bright sunlight, but can be cured within 2 days if the diseased is properly treated with the proper poultices. Some healers believe that the disease, once caught, can never be fully cured, but may result in flare ups many weeks or months later when the immune system is weakened by damage or stress, or when the sufferer spends a significant amount of time removed from sunlight.