CyberMaster

The CyberMasters were Cyber-converted Time Lords created by from the corpses of all those who had been on Gallifrey when he destroyed the Capitol. They retained the ability to regenerate, which made them effectively invincible.

The CyberMasters were apparently "destroyed" when Ko Sharmus detonated the death particle, obliterating all organic life on Gallifrey. (TV: The Timeless Children)