The Undying

 was a legend on Gelmoth. He was apparently immortal but had died. This was about 3000 years before Euphemia excavated him. Bernice Summerfield and the Unbound Doctor examined him and discovered that the the Undying was a fungus which possessed a man. The fungus made it's host charmed it's way through the civilisation. Arn burned the host removing the source of the spores. (AUDIO: The Undying Truth)