N is for... Nemesis

When a piece of living metal crashed to Earth in 1638, landing near the town of Windsor in England, it was found by the ruthless Lady Peinforte. She was an adept of the occult sciences, which she employed to achieve power and influence. She fashioned the silvery substance - validium - into a statue of herself in the form of Nemesis - the Greek Goddess who symbolised retribution and who punished those guilty of lack of respect towards the Olympians.

Validium originated on Gallifrey - invented in the Old Times as a sentient weapon of awesome power, designed to defend the planet. The statue was able to tell Peinforte something of its origins - and of the Doctor. 

He tracked it to Windsor 1638 and fought with Peinforte for control over Nemesis.

Winning the struggle, he launched the statue into space in a rocket sled after first removing its bow and arrow. This prevented it from achieving critical mass and reduced its influence. However, the rocket merely went into a slowly decaying Earth orbit. Each passing, every 25 years, resulted in troubled times on the planet below.

Lady Peinforte employed a mathematician to calculate when and where it would fall back to Earth - learning that this would be on 23rd November, 1988, just outside her home. She used her magic to travel forward in time with her manservant Richard to retrieve the statue - unaware that she was actually being manipulated by an ancient evil force known as Fenric.

Nemesis was also being sought by a Nazi war criminal - De Flores - and his army of mercenaries, and a force of Cybermen - all seeking to harness its potential for destruction.

It would regain its full powers once the bow and arrow were reunited with it. De Flores possessed the bow, and Lady Peinforte the arrow.

The Cybermen eventually won the statue. Refusing to accept this, Peinforte tried to physically possess it - only to be absorbed by it. The Cyber-Leader ordered Nemesis to rendezvous with its fleet, but the Doctor had already pre-programmed it to destroy the spaceships. 

Once this was done, it was to return to Gallifrey.

Played by: Fiona Walker. Appearances: Silver Nemesis (1988).

• One version of the myth of Helen of Troy has Nemesis as her mother, rather than Leda.
• Originally the punisher of those guilty of hubris, her name has come to refer to retribution for any crime.
• Whilst Walker, who portrays Lady Peinforte in the story, played the statue in some scenes, a polystyrene model was also used.
• Front-axial projection gave the statue its glow.
• It is suggested by dialogue that it was the Second Doctor who first encountered Peinforte and the statue in 1638. It's never explained how he came to have the validium in the first place - or why he allowed it to orbit the Earth for 350 years rather than getting it back to Gallifrey.
• Mention of it is conspicuous by its absence in any of the 21st Century episodes dealing with the fall of Gallifrey in the Time War.