ThreeEye is a supernatural drug first introduced in Storm Front.
It was manufactured and distributed by warlock Victor Sells; where and from whom he got the formula and knowledge for this complex potion is unknown, and is the first hint at the existence of a Black Council.(reference needed)
The drug enables users to gain a measure of the Sight, allowing them to see or sense supernatural presences that are lost to human eyes. One junkie is able to see the shadow of He Who Walks Behind around Harry Dresden.(reference needed)