Cybermen (Hell Bent), as there were two Cybermen: one Nightmare design, and one Cybus design
Talk about it here.
A Cyberman attempted to gain access to the Matrix hidden underneath the Capitol on Gallifrey during the Cloister Wars. It was attacked by the Cloister Wraiths, and it became a part of the Matrix hard drive. The Matrix attempted to use the Cyberman to attack Clara Oswald whilst she and the Twelfth Doctor travelled through the Cloisters, but the Doctor saved her. (TV: Hell Bent)