Eldrad was a criminal from the planet Kastria. A scientist, he built a protective field-the Solar Barriers- to protect his world against deadly solar winds, about 150 million years ago. He also designed the silicon bodies his race came to inhabit. However, he grew arrogant, and attempted to seize the throne of Kastria. When that failed, he removed the Barriers, forcing the Kastrian people underground.
Eldrad was sentenced to obliteration, but his hand survived and fell to Earth in the Jurassic Period. The hand was unearthed in the 1970s, whereupon he possessed several humans including the Fourth Doctor's companion, Sarah Jane Smith. Through them, Eldrad found his way to a nuclear power plant. Using its energy, he reconstructed his body, but under the influence of Sarah Jane, his new form was female.
Using lies and trickery, he convinced the Doctor to take him back to Kastria, now a dead world. An ancient trap wounded him, so he was placed in a device that regenerated him to his male form. However, he received a parting message from the dead Kastrian king, Rokon, who mocked him: "All hail Eldrad! King of nothing!" Enraged, Eldrad now threatened the Doctor and his companion, but was soon slain, for good.