Nurazh

The Nurazh were mind parasites originating in the Horsehead Nebula. The Third Doctor described them as "nasty".

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, 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. Once the Nurazh regained its strength, it betrayed the Master, having never intended to rule the planet. After briefly possessing Jo Grant, the Nurazh occupying the body of Matron Caxton fought with the Third 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. The two minds of the Doctor were too much for the Nurazh, and it overloaded. (PROSE: The Touch of the Nurazh)