Expedition to Telos

An expedition to the planet Telos was undertaken by the Brotherhood of Logicians. (TV: The Tomb of the Cybermen)

Dating
At the time of the expedition, Professor Parry believed that the Cybermen had been extinct for five hundred years. Indeed, John Viner recalled that the Cybermen had died out centuries prior. The Second Doctor noted that the Cybermen had been frozen for five centuries. (TV: The Tomb of the Cybermen)

The Celestial Intervention Agency dated the event to 2450, a point when the Earth Empire was growing and developing. (PROSE: A Sourcebook for Field Agents) ArcHivist Hegelia believed that this event took place "at the end of the 25th century", the same period in the CyberNomads waged their pursuit of Voga. (AUDIO: The Cyber Nomads) By another account, the event took place in the year 2503. (PROSE: The Cyber Files)

Background
As the CyberTelosian Controller recalled, the Cybermen were in danger of becoming extinct following the loss of their first planet, which was blamed on the Doctor. When the Cybermen's machinery stopped and our supply of replacements been depleted, they attacked the Moonbase only to be defeated by the Second Doctor, who was recorded in their history computer. (TV: The Tomb of the Cybermen)

Aftermath
ArcHivist Hegelia hypothesised that another group of CyberNomads came across Telos, reopened the Telosian tombs and helped to forge the new race of Cybermen, the Neomorphs, which proliferated during the 26th century. (PROSE: Killing Ground, AUDIO: The Early Cybermen)

During their attempt to destroy Earth in 2526, the Neomorph Cyber-Leader consulted the records of the Doctor. Among other incidents, he recalled the Second Doctor confining the Cybermen to their tombs on Telos. (TV: Earthshock)

On Telos, the Controller and his Cybermen advanced to Neomorph form. Seizing an alien time vessel, they attempted to destroy Earth and thus save Mondas in 1985 only to be thwarted by the Sixth Doctor, who destroyed the Cyber-Controller. This was followed by the destruction of Cyber-Control by the Cryons. (TV: Attack of the Cybermen)

Other references
Facing the Cybermen on a Mondasian colony ship, the Twelfth Doctor recalled Telos as one of the worlds where they rose, and that he had defeated them there by sealing them into their ice tombs. (TV: The Doctor Falls)

Behind the scenes

 * The Discontinuity Guide places the expedition as following the attack on the Wheel, which occurred no later than the early 22nd century, and preceding the 26th century Cyber-War.
 * Doctor Who: Cybermen states that the expedition occurred in the late 25th century circa 2486, before the pursuit of Voga in c. 2496. The remaining Nomads then combined with the Telosians to form the new Cyber-subspecies, the Neomorphs, around the year 2500.