They were beings of pure thought energy. An individual Nurazh could control thousands of minds at once. They travelled from world to world, consuming the mind energy of the beings on the world- simultaneously healing them of injuries- then leaving for the next planet.
The Master encountered a weakened Nurazh on a wasted planet. He was able to resist its control, and offered it a deal. The Master took it off the desolate planet, taking it to Earth, and it would give the Master an army of mind-controlled humans by possessing the residents of various nursing homes for wounded soldiers, healing their injuries. Once the Nurazh regained its strength, it betrayed the Master, having never intended to rule the planet but merely devour it, although the Master managed to escape. After briefly possessing Jo Grant, the Nurazh occupying the body of Matron Caxton fought with the Doctor. They both fell off the roof of Kenstone Hall. When Caxton died, the Nurazh tried to control the Doctor, but the fall had caused him to begin a regeneration. Attempting to control the Doctor's mind when he was technically in two minds at once proved to be too much for the Nurazh, and it died of the strain, although it was able to heal the Doctor's injuries and return him to his third body. (PROSE: The Touch of the Nurazh)