SRD:Dust of Tracelessness
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Dust of Tracelessness: This normal-seeming dust is actually a magic powder that can conceal the passage of its possessor and his companions. Tossing a handful of this dust into the air causes a chamber of up to 100 square feet of floor space to become as dusty, dirty, and cobweb-laden as if it had been abandoned and disused for a decade.
A handful of dust sprinkled along a trail causes evidence of the passage of as many as a dozen men and horses to be obliterated for 250 feet back into the distance. The results of the dust are instantaneous, and no magical aura lingers afterward from this use of the dust. Survival checks made to track a quarry across an area affected by this dust have a DC 20 higher than normal.