The CyberMasters were Cyber-converted Time Lords created by the Master from the corpses of all those who had been on Gallifrey when he destroyed the Capitol, claiming them all to have been "born" from the Doctor.
The CyberMasters externally resembled the Cyber-Warriors of the Cyber-Wars, though they had the ornate neckpieces of the Time Lords and Gallifreyan symbols on the side of the helmets. As such, they sported Cyber wrist blasters, which were capable of killing humanoids and even other Cybermen. Being Time Lords, however, they retained the ability to regenerate, which made them effectively invincible. (TV: The Timeless Children)
In response to discovering the lie of the Timeless Child which the Time Lords had kept hidden, the Master returned to Gallifrey and killed them all but made sure to preserve their bodies, keeping them "just cold enough" in the event they could be of use to him. That point came when a Cybercarrier from the aftermath of the Cyber-Wars passed through the Boundary to Gallifrey. Commandeering the resources of the Cybermen, the Master engineered a new race which he named the CyberMasters, Time Lord organics with the ability to regenerate and self-repair in Cyberarmour, making them invincible. Under his command, the CyberMasters were presented by the Master to the Thirteenth Doctor; the Master claimed that CyberMasters were "born" from the Doctor, who was originally the Timeless Child. Deeming that the CyberMasters could not be allowed to leave Gallifrey, the Doctor considered detonating the death particle, which had the power to obliterate all organic life on a planet, to stop them, but her place was taken by Ko Sharmus. On the Master's order, the CyberMasters repeatedly shot Ko Sharmus, who nonetheless detonated the death particle, destroying the Capitol. The CyberMasters were last heard from when the Master ordered them to follow him out through another exit way. (TV: The Timeless Children)