The Qetesh were a species that fed off excitement in people's lives, draining their life essence with it. They were masters of emotional manipulation and were also known as "soul-stealers".
The only known Qetesh is Ruby White, who tried to steal Sarah Jane Smith's soul. Ruby was imprisoned by her own race on a prison ship, with only a games console for entertainment, due to her uncontrollable hunger for the fear, excitement and life force of other beings. Over a long time she reprogrammed the games console into an AI to serve her needs, turning the prison into a working spacecraft. She went to many planets, draining and feeding off the inhabitants.
The Qetesh outwardly resembled humans, though they possessed far greater strength. Their stomachs existed outside their bodies. They did not need air to breathe.
The Qetesh imprisoned a member of their species, who took on the name Ruby White. When they placed the prison craft into space, Ruby escaped. She travelled to many planets, and drained them. She was recaptured by Sarah Jane Smith after the journalist defeated her, with help from her beloved team.