Eldrad

Eldrad was a silicon-based lifeform from the planet Kastria who saved his planet from solar winds, but then took down the barriers he created because the people wouldn't follow him. They sentenced him to destruction in an obliteration capsule, but his hand survived and ended up in a quarry on Earth. When Sarah Jane Smith came in contact with the hand, Eldrad controlled her, using her to bring his hand to a nuclear reactor. By absorbing radiation from the reactor, Eldrad was regenerated in a thin female body with violet skin covered in crystals, basing his form on his contact with Sarah Jane. He convinced the Doctor to take him back to Kastria in the present, where he regained his true form, only to learn that his race had destroyed themselves to prevent him from ruling them again. The Doctor and Sarah used the Doctor's scarf to trip Eldrad and send him falling into a pit.