Parallax is a demonic parasitic entity dating back to the dawn of time. This parasite was the sentient embodiment of fear, traveling from world to world and causing entire civilizations to destroy themselves out of paranoia.
The Guardians of the Universe imprisoned Parallax within the central Power Battery on Oa using fear's opposite energy, willpower. Parallax had lain dormant for billions of years, its true nature hidden by the Guardians to prevent anyone from trying to free it. Being yellow in color, Parallax eventually came to be referred to as simply "the yellow impurity."
This was the reason why the rings were useless against the color yellow: Parallax weakened its power over the corresponding spectrum;hence only someone with the willpower to overcome great fear could master the power ring. Thus, when recruiting new Green Lanterns the Guardians were careful to look for recruits that were largely fearless. When the renegade Sinestro was later imprisoned in the Power Battery himself, his Qwardian yellow power ring tapped into Parallax's power and awakened it.
Since Sinestro hated Hal, Parallax chose Jordan as its tool to free itself. Parallax spent years influencing Hal, causing him increasing self-doubt and notably his premature whitening at the temples. Parallax's control over Jordan became nearly complete with Jordan's grief over the destruction of Coast City, and it was Parallax who was responsible for Jordan's subsequent murderous activity, his apparent killing of Sinestro (which was later revealed to be an energy construct created by Parallax and Sinestro's manipulation to the creature's creation with his own Qwardian ring, created as the final stage of Jordan's susceptibility to the impurity in order to break his will), and Jordan's destruction of the Central Battery, which allowed Parallax to graft itself onto Jordan's soul.
Despite Parallax’s seeming conquest of Jordan’s soul, Jordan’s heroic tendencies managed to manifest themselves enough for him to sacrifice his life in order to reignite Earth’s Sun in the Final Night. Jordan’s soul (with Parallax still grafted onto it) subsequently became the newest host of the Spectre, the vengeful spirit of God’s wrath.
Since Parallax was gone from the Oan Power Battery, the final power ring conferred to Kyle Rayner did not have any weakness against yellow. The Spectre eventually ejected Parallax from Jordan's soul, then departing in order to move onto the next recipient of the Spectre, while Ganthet guided Jordan's soul back to his own body, preserved after Jordan reignited the Sun.
His soul and thoughts finally clear for the first time in years, Jordan was resurrected, again taking his place as a Green Lantern; even regaining some of his lost youth (evidenced by his hair becoming brown again).
Together, Jordan and Kyle Rayner, along with fellow Green Lanterns John Stewart, Guy Gardner and Kilowog freed Ganthet, whom Parallax had possessed after his expulsion from Jordan's soul, and imprisoned the parasite back in the Central Power Battery on Oa.
Despite this re-introduction of the "yellow impurity", which is now referred to as the Parallax Fear Anomaly, the power rings' weakness against yellow no longer applies, as experienced wielders are now able to consciously recognize its source, and overcome the fear associated with it. It is still, however, a considerable weakness for new GL recruits, who are ignorant of the impurity's nature or of how to combat it. During training, new recruits' rings may falter at times against yellow objects, and/or when a recruit is experiencing panic or fear.
Recently, the Sinestro Corps captured Kyle Rayner with the intention of making him Parallax's new host. Kyle has his powers drained out of him by Sinestro himself, and is immediately taken over by Parallax. Parallax then clothes itself in a new uniform which appears as a combination of the Sinestro Corps' uniform, Kyle's original Green Lantern costume and the armor Hal Jordan wore as Parallax and returns to Qward with the Sinestro Corps. Parallax is made a herald of the reborn Anti-Monitor.
The Forms of Parallax:
The Parallax Entity
Parallax I
Parallax II
Parallax III
