Eldrad's ring

Eldrad developed a ring which contained a crystal imprinted with his genetic code. He considered it to be his "key to eternity" as it allowed him to revitalise his physical form from any radioactive source.

After his execution in an obliteration module, the ring remained on the hand that survived and ended up on Earth. There, Sarah Jane Smith found the hand and took the ring. She used it to render Doctor Carter and several workers at the Nunton Experimental Complex unconscious under the influence of Eldrad. The Fourth Doctor took her away from he ring and Driscoll, a worked at the complex, found it. He used it incapacitate those workers who tried to stop him taking the hand into the reactor core.

Eldrad was restored in a female humanoid form. On Kastria, with Eldrad weakened by an acid-laden booby trap, the Doctor took the ring and used it to open a door and activate the regenerator chamber. Eldrad was retuned to his male form. After requesting to be returned to Earth, he asked that the Doctor give him his ring. The Time Lord distracted him, by throwing his magician's stick. Eldrad was tripped into an abyss by the Doctor and Sarah and the former dropped the ring in after him. (TV: The Hand of Fear)