Blanks

"The Blanks" was the name the Twelfth Doctor gave to the alien-robots from another planet. They were able to take on the appearance of anyone they choose. After taking on human forms themselves, they were tasked to recreate a more civilized society on Earth, as they have done on other planets before. They were also able to provide their form with eternal youth, able to keep their bodies young.

History
The Blanks had been on Earth since humanity's beginning, secretly manipulating humanity and its development for their own ends. They experimented on and dissected humans, and eventually learned how to exist into the human-form themselves.

The Doctor learned that their mission, given to them by the pilot, is to recreate a more civilized society on Earth, as they have done on other planets before. They believe Earth is the most uncivilized planet in the galaxy.