Wondering Monster

The Geron is an elemental demon made of evil moon dust, appearing most often in the form of a seven-foot-tall humanoid with long, tapered arms and legs, floating a few inches above the ground, with a blank dark space instead of a face, where a strange silvery light occasionally flickers.

If a living creature is touched by the Geron, it is dealt 1d6 damage and must succeed in a DC 16 Constitution saving throw or begin turning to loose, dusty stone (losing 1d8 maximum HP at the end of each round). A creature who reaches 0 HP in this way turns completely into dust.