I took the liberty of backing out of original, because I suspect, that model makers just chose an easy way and used one of old CG models (obviously, model of prisoner), and added flame effects to it.
Fire shadows inhabit boundary regions between dimension of the dead and elemental dimension. This is condition of creatures, who has died in flames. Some of creatures who die such death not just pass away, but get projected on streams of elemental energy. It is endless journey between dimensions. Living people perceive this as unceasing spasmodic movings, appearings and disappearings. Sometimes you can see just separate body parts - hands, faces... Fire shadows transform every second.
You can find them only in deserted places, to which they was attached in life. Even other undead creatures avoid them. They almost don't think and never communicate. Their life is permanent agony.
Date: October 2006.