From Aufstrag

The discarnate are lost souls, shadows of their former selves. They linger in the infernal planes as a wisp of pale, gray smoke. Much like the smoke from any fire the discarnate are tangible, but not so. They cling to any living tissue they come into contact with, coalescing in coils of evanescent smoke. The discarnate are cold, to pass through him draws the warmth from living tissue, but they are mindless and see little beyond their own suffering.

The discarnate are the souls and spirits of those who died in hell and were trapped there, their bodies left to rot, unburied. The discarnate cannot leave hell for the weight of that dread place bares down upon them. Lost in fear they shed what they were and became nameless voices of despair, lost in the nether world. There they gather like some foul gas, congealing into a noxious poison, often settling in pits and deep holes where they became a mass of twisted fear.

The discarnate are barely cognizant of anything around them, though living creatures attract them as a haunting memory of what they once were. The naturally drift toward them, hoping to engulf them in order to make themselves whole again. Of course they cannot, and all they manage to do is choke to death the person with whom they sought to join.