The Sentient sun Umbrax traverses unseen through space, drawn toward planets where self-destructive forces are strongest. It surrounds these worlds encasing them with its energy and power, and then yanks them into its galaxy, adding it and it's inhabitants to the Ultraviolet Lantern Corps.
The first recruit to the new corp was Green Lantern John Steward of Earth. Sinestro had believed that John would have a rapport for the darker side of the unseen light. So Thaal Sinestro used John Stewart to lure Umbrax to Earth.
The Corps itself is fully controlled by Umbrax, rather than each corp member controlling their own emotions; though this is not always true. Sinestro found a way to bind and isolate the Ultraviolet power into himself, using his original costume to distill the energy of Umbrax without succumbing to its mind control and he and now helms the Ultraviolet Corps. John Stewart was also able to overcome the influence and power of Umbrax and use the Ultraviolet energy through his own will and confronting and defeating the negative emotions he had deep inside of himself.