Sinestro
Sinestro is the only member of the Green Lantern Corps to turn evil. Stripped of his power ring, Sinestro was banished to the anti-matter universe of Qward. There, he partnered with the Weaponeers of Qward, who gave him his yellow power ring. Sinestro has been a continual threat to Green Lantern Corps, swearing to destroy them and conquer the universe along the way.