Mind Fog is a mind-controlling mist. It is first seen in Summer Knight.
In the series
In Summer Knight, when Harry was meeting with Karrin Murphy for support on his case involving the murder of the Summer Knight, Ronald Reuel, at Wal-Mart, the store was attacked by the Mind Fog. Clerks and customers sagged unconscious. Harry drew a Circle around himself and Murphy to block the mist using salt from a shaker he had snatched from the Wal-mart snack booth.
While inside the Circle, Harry tied a string improvised from a stray thread and Murphy's hairs around Murphy's finger. Then he suffused it with a memory Charm—short-term magic—"Memoratum, defender memorarius"—to block the mist from entering her and the string is to help Murphy remember so that they can leave the circle and get out of the store past the Mind Fog.
Shortly after the events of Summer Knight, Dresden and Bob researched the working of the Mind Fog spell.