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)
The CyberMasters were apparently destroyed when Ko Sharmus detonated the death particle, obliterating all organic life on Gallifrey, with the last they are heard from being when the Master ordered them to follow him out through another exit way. (TV: The Timeless Children)