The Sinestro Corps

“In blackest day, in brightest night, Beware your fears made into light. Let those who try to stop what's right, Burn like his power... Sinestro's might!”

That is the oath of the Sinestro "Yellow Lantern" Corps.

Trapped within the Central Power Battery of Oa was a space parasite known as Parallax. Parallax instilled great fear and was the reason for the impurity in the Green Lantern rings that rendered them useless against the color yellow.

Sinestro was the mentor to Hal Jordan and one of the greatest of all the Green Lanterns. Sinestro never really saw eye to eye with the Guardians and used his power ring to instill great fear into the hearts of those who did injustice. The Guardian's banished Sinestro to the Animatter Universe. It was on the planet Qward in which Sinestro joined forces with The Weaponers. He forged a yellow power ring and the Weaponers sent him back to our universe to seek revenge on the Guardians.

After being defeated, Sinestro returned to Qward and started to forge enough rings to make his own Corps and defeat the Green Lanterns once and for all. Due to the fact that the Sinestro Corps were so wide spread and killing so many GL Corp members, the Guardians had no other choice that to rewrite the Book of Oa and grant the Green Lanterns the use of lethal force on any member of the Sinestro Corp during "The Sinestro Corps War".