6. Parallax
The yellow impurity, the reason for the Lanterns' weakness against yellow (fear) and the driving force behind Hal Jordan's decent into murder and madness. Each Lantern Corps had a living entity and for the Sinestro Corps, the yellow power of fear was embodied by Parallax. The destruction it has been responsible for is almost incalculable, Jordan was not the only Lantern to be possessed by the creature, poor Kyle Rayner was run through the ringer with the kind of mental cruelty only the very best villains can heap upon their enemies. As one of the biggest catalysts in Lantern stories over the past two decades, Parallax must be regarded as a central figure. The destruction of the Corps (through Jordan,) the spark behind the killing sprees of the Sinestro Corp, it has done a great deal of damage to our heroes. Fear will never die, neither will Parallax.