Telos

Telos was a planet, the homeworld of the Cryons until it was colonised by the Cybermen as a tomb world following the destruction of Mondas, (TV: Attack of the Cybermen) with some accounts even holding that one race of Cybermen had been native to Telos already. (PROSE: Doctor Who and the Cybermen) It was known to UNIT as one of the home planets of the Cybermen. (PROSE: Cybermen)

The CyberTelosians placed themselves in suspended animation in their Cyber-tombs for centuries until an archeological digging revived them in the presence of the Second Doctor. (TV: The Tomb of the Cybermen)

Characteristics
Telos was located outside of Mutter's Spiral (AUDIO: Last of the Cybermen) in Tremulus Three, near Tasker's Crown (PROSE: Attack of the Cybermen) and also near the star system containing Krelos. (AUDIO: Return to Telos)

A world that began with "Tel", before Red Rocket Rising's Acting President Eileen Klint's message was cut off, existed on the edge of Red Rocket Rising's solar system. (AUDIO: Blood of the Daleks)

Following Cyberman occupation, Telos was a grey, barren, and rocky world. Diamonds, vastial and other minerals could be found on the planet in abundace. (TV: Attack of the Cybermen)

Descent of the Cryons
At one time Cryons, who died instantly when exposed to temperatures above zero degrees, lived on Telos. If the Cryons ventured onto the surface, they would boil. The Cryons moved under the surface when it became too warm, constructing underground cities refrigerated to the sub-zero temperatures the Cryons needed to survive. (PROSE: Attack of the Cybermen)

Ascendancy of the Cybermen
According to some accounts, a "race of men" on Telos sought immortality through the art of cybernetics and became the first Cybermen. Developing space travel, these Telosian Cybermen created an empire feared throughout the galaxy. (PROSE: Doctor Who and the Tomb of the Cybermen, Doctor Who and the Cybermen) They conquered the planet Mondas only for it to be destroyed as it neared Earth, (PROSE: Doctor Who and the Cybermen) and, after their defeat on the Moonbase in 2070, they retreated into their Cyber-tombs where they slumbered in suspended animation for centuries. (PROSE: Doctor Who and the Tomb of the Cybermen)

According to another telling, the Cybermen originally developed in this manner on Mondas, (PROSE: The Creation of the Cybermen) though most accounts agreed the CyberMondans developed more out of a desire to survive. (AUDIO: Spare Parts) After the destruction of Mondas, the Cybermen hid out on the planet Lonsis, planning to invade Earth. They were foiled by Lucie Miller, who infected their ship with temporal corrosion, and later the Doctor, who increased the range of a Quantum Crystalliser programmed to destroy them. The few survivors of these attacks colonised Telos, while Cybermen from other parts of the universe colonised other planets. (AUDIO: Human Resources) According to these accounts, the Mondasian Cybermen selected Telos with the intent of taking over the Cryons' refrigerated cities to use as repositories of Cybermen cryonically frozen in suspended animation. The Cybermen killed most of the Cryons. Some survived, though not many. (TV: Attack of the Cybermen) According to a variant account, the colonisation of Telos by the Mondasian Cybermen predated the destruction of Mondas. (PROSE: The Cyber Files)

The Twelfth Doctor later acknowledged Telos and Mondas as unconnected and equally-primal Cybermen homeworlds, arguing that the emergence of nearly-identical Cyber-races on the two planets was a case of parallel evolution rather than the effect of some "masterplan" by an "evil genius". (TV: The Doctor Falls)

As a Cyber-planet
At any rate, Telos was a Cyber-bastion throughout the Cyber-Wars. Earth forces launched a final assault on Telos towards those conflicts' end and the planet's surface was bombarded. (AUDIO: Last of the Cybermen)

Five hundred years after the last known encounter between humanity and the Cybermen, an archaeological expedition from Earth, led by archaeologist Parry (but financed and masterminded by Kaftan and Eric Klieg of the Brotherhood of Logicians), landed on the planet to find the Cybermen and to revive them. With the Cybermen as their army, the Brotherhood planned to impose a regime of pure logic on human society. The Cybermen and their leader, the Cyber-Controller, were resurrected. The Second Doctor was able to seal them back inside their "tombs" and to electrify the outside gates. (TV: The Tomb of the Cybermen) This event was recorded under Awakening of the CyberTelosians, one of the Cyber-Documents. (AUDIO: The Cyber Nomads)

Cyber-departure from Telos
The Cyber-Controller was not killed, as the Doctor had believed, but survived, and had installed a base, known as Cyber-Control underground, supervising operations in 1985 London, with which they had a communications link, and fighting the Cryon rebels who remained. The Cybermen at this time employed work parties of humans whose conversions were unsuccessful as slaves to mine the surface of the planet. The Cybermen intended to destroy Telos and to observe its blast pattern for future research. The base was, however, invaded and destroyed by the Doctor (now in his sixth incarnation) with the assistance of the captured Cryon leader Flast. (TV: Attack of the Cybermen) This event was recorded under Departure from Telos, one of the Cyber-Documents. (AUDIO: The Ultimate Cybermen)

Persistence of Cybermen on Telos
The Eleventh Doctor later revisited Telos while trying to save Billy Jones, who had been scattered through time. At this point in time, the Cyber-tombs had returned to their multi-tiered appearance, and had become occupied by a different Cyberman variant, the Cybus-type Cyber Legions model, who began to wake from the tombs. (GAME: The Mazes of Time)

The Twelfth Doctor sent "Lumpy" the Dalek there to retrieve one of the pieces of the Orb of Fates, which was hidden in some ice. Once the piece was taken, the weapons-grade Cybermen attacked Lumpy. (GAME: The Doctor and the Dalek)

Eventual fate
At some point during the early 26th century, Telos was destroyed by an asteroid impact. (AUDIO: Telos)

Other realities
In one of the infinite parallel universes of "possible space", (COMIC: Fire and Brimstone) the last of the Cybs were forcibly interred on Telos after their planet Mondas was finally destroyed. (PROSE: The Chronicles of Doctor Who?)

Other references
The Second Doctor offered to show River Song "this cool place on Telos." (GAME: The Eternity Clock)

The renegade Time Lady Iris Wildthyme once claimed to have visited the Cyber-tombs on Telos with her assistant Jeremy. (PROSE: The Scarlet Empress)

After the Hotel Historia on Earth was invaded by Graxnix, manager Majenta Pryce decided to flee and relocate to somewhere like Telos, claiming that it could use a little remodelling. (COMIC: Hotel Historia)

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)

A joke book which the Thirteenth Doctor and Yaz once found themselves trapped in contained an apparent transcription of a conversation between 2 individuals. The first claimed to have found a planet with a big tomb of Cybermen. The second asked "Telos?", to which the first said they'd tell everyone. (PROSE: Knock! Knock! Who's There?)