Fendahl

The Fendahl were a gestalt creature consisting of a core and twelve Fendahleen, which are the same as the giant slugs on a smaller scale. The Fendahl feeds and grows on death. It was believed to have perished in the destruction of the Fifth Planet, but a skull containing the Fendahl's dormant energy landed on Earth where it was discovered 12 million years later. The Time Scanner reactivated it and it took over Thea Ransome as the core. It then went on to make Fendahleens from other humans. The Doctor destroyed the Fendahleen through the use of the skull in a Super Nova.

Professor Fendelman was a scientist infected by the influence of the Fendahl. He innocently invented the Time Scanner,  which cause the whole chain of events to take place.

The Fendahl were only in The Image of Fendahl