They are said to serve someone or something called the Silent One.
Strange emaciated humanoids dressed in rags and with deathly pale skin, the sandmen possess pitch black eyes, spider-like fingers and eerily silent movements.
Those meeting them tend to forget they ever saw them. They seem to have an ability to damage or destroy minds. Photographic equipment only reveals blurry images of them.
Its not clear that they are alive in a normal sense. Bullets and impact weapons hurt them but weakly, causing purely physical damage and small wounds that bleed dust. But they can die. Water and fire seem to be capable of destroying their desiccated bodies. They are filled with a strange sand or dust that has properties never recorded in science.
According to the Cheiron Group they subsist on memories, and that if they drain enough memories they can turn another person into one of them. They are extremely rare elsewhere in the world. Typically fairly dumb, the ones in Seattle are oddly organized and intelligent.