Cybermen

The Cybermen originally evolved from the inhabitants of Mondas, Earth's twin planet.   The Mondasian civilization developed more quickly than Earth's and Mondasians may have visited our planet in prehistoric times.  At one point in ancient history, for reasons yet unknown, Mondas left the Solar System.  It was during this period of wandering that the Mondasians evolved into creatures of ruthless logic; their organs replaced by cybernetic parts until they became Cybermen.  The Cybermen were virtually immortal, their only recorded weaknesses being gold (which crippled their breathing mechanisms) and high susceptibility to gravitation and certain forms of radiation.   The Cybermen have evolved into numerous forms, including that of Cybercontrollers.

It has been surmised that, at a certain point during their space journey, some Cybermen left Mordas to form a planetary empire, enslaving other civilisations and turning them into Cybermen.  This empire included planets such as Planet 14.  Eventually, these "galactic" Cybermen began to feel threatened by Eath's technological advance and forays into outer space.  They launched an invasion of Earth in 1970, with the help of Tobias Vaughn, head of International Electromatics.  They recognized the Second Doctor as a hostile alien from a so-far unrecorded encounter on Planet 14.   The Doctor and a newly former UNIT thwarted their plans, and most of their space fleet was destroyed.  In 1988, the same Cybermen launched yet another attack against Earth, but this time, they were defeated by the Seventh Doctor; the remains of their fleet were destroyed by the Silver Nemesis.

Meanwhile, two years earlier, in 1986, Mondas had returned to the Solar System.   Its Cybermen planned to steal Earth's energy to replace their own depleted resources.  The Mondasian Cybermen launched an attack on the planet, planning to drain it of its energy to repower Mondas.  They were thwarted at the South Pole by the First Doctor.  Mondas absorbed too much energy and was destroyed.  Cybermen from the future, using a hijacked time vessel, secretly traveled back in time to 1985 and tried to crash Halley's Comet into Earth to save Mondas.  They were defeated by the Sixth Doctor and Lytton.

In AD 2030, the galactic Cybermen launched another attack against Earth, this time planning to use the Wheel in Space.  In 2070, they invaded the Moonbase.  In both cases, they were defeated by the Second Doctor.

Finally, Earth's expansion into space brought mankind in direct conflict with the Cybermen.  Humanity found itself on the side of those alien species who had been fighting the Cybermen, and formed the Alliance.  During the First Cyber Wars, the Alliance discovered that gold, which exists in abundance on Voga, was fatal to their enemies, and begun using glitterguns.  Driven from their worlds, the Cybermen had no choice but to flee.  Before they did, they made an attempt to destroy Voga, but were only partially successful.  A fragment survived and began to drift towards the Solar System.  Soon afterwards, the Cybermen conquered the planet Telos and drove its native Cryons underground.  They turned the Cryons' cities into "Tombs",   in an attempt to heal and preserve their species for future assaults.

Towards theend of the 25th-century, the Fourth Doctor, Sarah Jane Smith, and Harry Sullivan thwarted the efforts of a small band of isolated Cybermen who were trying to destroy the remains of Voga, which had drifted near Space Beacon Nerva in Earth's Solar System.

At the beginning of the 26th-century, the insane Logician Klieg attempted to resurrect the Cybermen, Whose Tombs had been located on Telos.  His plans were foiled by the Second Doctor, Jamie, and Victoria Waterfield.  Once the Tombs had been opened, either the Cybermen continued to be reactivated, or other Cyber Forces drifting in space were summoned to complete the job Klieg had begun.  In any event, the Cybermen had time to regroup, and reemerged as a serious galactic threat.

In AD 2526, made aware of this new Cyber menace, the Alliance agreed to meet on Earth in a Galactic Congress to decide what steps to take.  The Cybermen then attempted to destroy the COngress by causing a space freighter to crash on the planet, but their scheme was thwarted by the Fifth Doctor and his companions, resulting in the death of Adric.

A second series of Cyber Wars ensued.  Having found Voga, the Alliance was again in possession of a vast supply of gold.  The result was yet another defeat for the Cybermen.  Just before the Alliance's forces made a final attack on Telos, the Cybermen's stronghold, the Cybermen used a hijacked time vessel to launch a final, desperate attack.  They tried to not only save themselves, but alter the course of history.  The time vessel may have been of Morok origins, as they were one of the few species with limited time travel capabilities at that time.  However, the Cybermen were defeated by the Sixth Doctor and the courageous Cryons.

At one point in time, a crew of  galactic Cybermen were used by Borusa as pawns in the Game of Rassilon.