Kyle Rayner was chosen to wield the last ring because he knew fear, and Parallax had been released from the Central Power Battery. Ganthet knew this and chose Kyle because his experiences dealing with fear enabled him to resist Parallax. Because Parallax is a manifestation of fear, and yellow, none of the other Green Lanterns, including Hal, could harm Parallax and, therefore, came under his control. Kyle taught them to feel and overcome fear so they could defeat Parallax and incarcerate him in the Central Power Battery once again.