The CyberMasters were a race of Cybermen created by the Spy Master from the corpses of the Time Lords who had been on Gallifrey when he destroyed the Capitol, claiming them all to have been "born" from the Doctor.
The CyberMasters possessed Cyber-bodies very similar to those of the Cyber-Warriors that Ashad reawakened in the aftermath of the Cyber-Wars of the Cyberium, though they possessed several notable differences.
The Cyber-helmets of CyberMasters had large, silver Time Lord headdresses, with Gallifreyan symbols on the sides and the outer layer of their faceplates, in place of a pair of side handles. The CyberMasters' faceplates also featured "tear-drop" eye-pods that angled vertically, rather than diagonally, and their mouth slits were grated frowns. In addition, each CyberMaster wore a long, black robe with Gallifreyan symbols etched down the inner front sleeves. (TV: The Timeless Children) The voices of CyberMasters were highly reminiscent of the voices of the early Cybermen of Cybus Industries. (TV: Rise of the Cybermen, The Age of Steel) The CyberMasters possessed Cyber wrist blasters identical to those of the Cyber-Warriors they were based upon; several dozen blasts from these wrist blasters could mortally injure a human, with the injuries sustained leading to death within a second.
The most striking characteristic of the CyberMasters was their ability to regenerate; the Spy Master converted these Cybermen from the corpses of Time Lords and genetically spliced the Time Lords' regenerative abilities into them. This effectively gave the CyberMasters an advanced self-repair system, allowing them to regenerate and recover from otherwise-fatal damage. The Master proclaimed that this made the CyberMasters "invincible", and demonstrated to the Thirteenth Doctor the CyberMasters' regenerative powers by having one CyberMaster shoot another CyberMaster with its Cyber wrist blaster; the latter CyberMaster was temporarily terminated, but quickly regenerated and fully restored itself. However, as they contained organic components, the CyberMasters were still vulnerable to the death particle, even after it had been shrunk by the Master's Tissue Compression Eliminator.
Anomalously, although the CyberMasters, being a Cyber-subspecies, were programmed to be emotionless, they would not immediately obey an order if it would result in damage to a fellow CyberMaster; indeed, when the Master demonstrated the CyberMasters' regenerative capabilities to the Doctor, the CyberMaster he ordered to shoot at a fellow CyberMaster was initially reluctant to do so, until the Master angrily ordered it to comply. (TV: The Timeless Children)
When the Spy Master hacked into the Matrix on Gallifrey, he accidentally discovered the story of the Timeless Child; furious that the entirety of Time Lord civilisation had been built on "a lie", the Master destroyed the Capitol and committed genocide against the Time Lords. (TV: Spyfall) However, he chose to preserve the corpses of the Time Lords, believing them to be of use someday. (TV: The Timeless Children)
Sometime after escaping the Kasaavin realm, (TV: Spyfall, Fugitive of the Judoon) the Master passed through the Boundary when it opened between its planet and Gallifrey, (TV: Ascension of the Cybermen) and took the Thirteenth Doctor back to the ruins of the Citadel. While the Doctor was trapped within the Matrix, the Master welcomed Ashad to pilot his Cybercarrier to Gallifrey. Inside the heart of the Capitol, the Master, disappointed by Ashad's plan to transform the Cybermen into a purely robotic species and then use the death particle to destroy all organic life, persuaded Ashad into an alliance with him, claiming that doing so would make the Cybermen the true dominant species of N-Space. Onboard the Cybercarrier, the Master killed Ashad and absorbed the Cyberium after it left Ashad's corpse; with their combined knowledge of the Cybermen's entire history and the Matrix, the Master and the Cyberium converted the Time Lord corpses frozen in stasis by the Master into the CyberMasters, before returning to the heart of the Capitol.
Inside the heart of the Capitol, the Master awoke the Doctor and unveiled twelve of the CyberMasters to her. In addition, he ordered one CyberMaster to shoot another, demonstrating the CyberMasters' regenerative abilities. Declaring the CyberMasters to have been "born" from the Doctor, and that their creation was "a new era dawning", the Master and the CyberMasters departed.
For Gallifrey. For the Time Lords. For the end of the universe itself!
The CyberMasters and the Master were later confronted in the heart of the Capitol again by the Doctor, after she broke out of the Matrix and found Ashad's shrunken corpse onboard the Cybercarrier; the CyberMasters attempted to shoot the Doctor on sight, but were called off by the Master. Armed with one of Ko Sharmus' explosives, with Ashad's miniaturised corpse attached, the Doctor attempted to detonate the explosive and, by extension, the death particle within Ashad's corpse, to destroy the CyberMasters, along with herself and the Master. However, after failing to detonate the bomb out of hesitancy, Ko Sharmus arrived, having chosen not to escape in the spare TARDIS used by the rest of Team TARDIS, Ravio, Yedlarmi and Ethan. (TV: The Timeless Children) Revealing that he had been part of the Alliance, (TV: Fugitive of the Judoon) who had sent the Cyberium to Earth in 1816, (TV: The Haunting of Villa Diodati) Ko Sharmus persuaded the Doctor into letting him detonate the death particle while she escaped in another TARDIS. Enraged, the Master ordered the CyberMasters to kill Ko Sharmus; despite being shot dozens of times in only a few seconds, Ko Sharmus detonated the explosive, thereby detonating the death particle. The Master ordered the CyberMasters to escape with him, immediately before the death particle exploded. (TV: The Timeless Children)