A very dark thing for certain, and to me quite a poignant one. I believe this is an ambrotype, and certainly its is a Civil war soldier in Union uniform captured in the image, whose head, through some combination of damage and the effects of time, I assume just chance occurrence, appears transformed into a cloud of smoke. About as haunting a portrait, and portrait of the effects of war, as I might imagine.
Sold as found in case (missing opposite side) which measures 3 3/4" x 3 1/4 and shows wear around the edges but is in pretty good, stable and sound, condition. I have not taken it out of the case. I have shot it in a few different lighting conditions to try to give a good sense. Some glare caught in photos.