Operation Exodus

Operation Exodus was, according to, the residents on the lowest floors of the Mondasian colony ship's last chance to survive after several hundred years of decay because of the time differential caused by a black hole.

The plan resulted in the residents converting themselves into Cybermen to survive long enough to climb to the higher floors, with early prototypes able to make trips up in the elevators. Several of these prototypes made it as high as Floor 0507, the first successful mission. Others would later be able to climb as high as Floor 0000, but could not stay for long, as the trip lasted several years for those on the lowest floor. (TV: World Enough and Time)

The Master initially took advantage of this, inserting himself in the local hospital, spending several years there, controlling them. He also ensured Bill Potts was converted when she was brought below. (TV: World Enough and Time) However, he lost control when the Twelfth Doctor altered the controls, turning the Cybermen against him. The Cybermen independently continued the task, pushing upwards until they were defeated by the Doctor on Floor 0507. Though most of the Cybermen were destroyed in this conflict, Nardole predicted that the survivors would eventually regroup and come after the human colonists again, but expected that they had been bought enough time for him to figure something out when it did happen. (TV: The Doctor Falls)