- "I never fathomed that the impurity was alive. That my Qwardian ring was tapping into sentient fear." --Sinestro talking about Parallax.
After they created the Green Lantern Corps, the Guardians of the Universe imprisioned Parallax within the Green Lantern Central Power Battery. There, Parallax was continually held in check by the contained energy of Willpower, and the emotional entity Ion itself. The Guardians never spoke of Parallax to the Green Lanterns, and kept it hidden from all others in order to prevent anyone from trying to free the fear entity.
Over time, the dormant Parallax became known as the "yellow impurity", as Parallax's hibernating essence was partially dissipated into the many Green Lantern Power Batteries and thus later into the Green Lantern Power Rings used by the Corps. Since Parallax was yellow, its weakened essence caused the Power Rings used by the Corps to be ineffective against objects that were also yellow. The Guardians explained this to their Lanterns as an unavoidable flaw within the Power Rings.
After Hal Jordan destroyed the Green Lantern Central Power Battery and the Green Lantern Corps, Parallax was released from Oa. Only Kyle Rayner remained of the Corps, and because Parallax was freed, his ring was immune to the "impurity". When the Green Lantern Central Power Battery was rebuilt, the residual effects of Parallax's long-term imprisionment remained, but with a critical difference: if a Lantern could confront and conquer their fears, then they could overcome the "impurity".