The Yellow Impurity was formed when the Guardians of the Universe imprisoned the fear elemental known as the Parallax Entity within the Green Lantern Central Power Battery. This act left the creature of the Emotional Spectrum dormant and thus ended its reign of terror but also introduced the yellow impurity within the Green Light of Will. The taint meant that the green light was vulnerable against items or individuals coated in the color yellow and thus meant that such things were one of the only weaknesses of the Green Lantern Power Ring. After the rebellion of their Manhunters, the Oans also coated their city on their homeworld with yellow in order to prevent their sentient servants in the Green Lantern Corps from rebelling against them.