Nemesis

The Nemesis, also known as the Statue of Nemesis and, upon the rocket-sled, the Comet Nemesis, was a statue of validium that was capable of great destruction. Before arriving on Earth, the Nemesis had other forms that would horrify a person and had other names as well. It was also used by the Doctor once before, and only wanted her freedom. Since the statue was of validium, it could reform after it had been destroyed. (TV: Silver Nemesis)

History
When the validium did arrive on Earth in Windsor in 1638, it was found by Lady Peinforte, who shaped it into a statue of herself holding a bow and arrow. Before she could use its power, the Doctor, knowing its power, launched it on the rocket-sled into space, without the bow and arrow.

Upon entering space, the Nemesis entered an orbit which brought it back to Earth every 25 years, bringing destruction. It came in 1913 on the eve of the First World War, when it was witnessed by a soldier named Wrightson, (PROSE: Human Nature) it came in 1938 when Adolf Hitler annexed Austria, and it came in 1963 when President John F. Kennedy was assassinated.

The Nemesis finally returned to Earth in 1988 and was fought for by Lady Peinforte, who time travelled from 1638 to 1988; De Flores, a neo-Nazi wishing to start the Fourth Reich; and the Cybermen wanting to conquer Earth with the combined forces of the Nemesis and their Cyber-Fleet and turn the planet into New Mondas. All the while, the Seventh Doctor was trying to stop them.

In order for the validium of the statue to be activated, it required a critical mass, so the statue needed the bow, which was brought by De Flores, and the arrow, which was brought by Lady Peinforte. Eventually, the Doctor retrieved the bow and then had to deal with Peinforte and the Cyber-Leader, De Flores having been killed by the Leader. Lady Peinforte threatened to tell the Cybermen of the Doctor's secrets, of the Old Time on Gallifrey that the statue told her.

The Doctor surrendered the bow to the Cyber-Leader, and insisted Lady Peinforte tell them. She started to, only to be stopped by the Leader's intrusion: "The secrets of the Time Lords are of no interest to us." With the rocket-sled about to launch the Nemesis to meet with the Fleet, Lady Peinforte went mad and jumped into the sled, merging with the statue just before it launched into space.

The Nemesis struck across space towards the Cyber-Fleet in orbit around the Moon. Upon entering the Fleet, the Nemesis exploded, destroying the entire Cyber-Fleet. (TV: Silver Nemesis)

In an alternate timeline, the Doctor attempted to destroy the Cyber-Fleet with nemesis mines before being confronted by a cyber-converted Ace. (COMIC: Prologue: The Seventh Doctor)