Sleeper (Last Christmas)

The sleepers were dream crab induced metaphorical constructs inside a shared dreamscape which visually resembled the dream crabs' victims in the middle of an attack by one of the dream crabs. They represented the part of the dreamer's mind which had surrendered to the dream crab attack, and were images of what was coming to kill the dream crab's victim as they dreamt. In one shared dream, sleepers took on the form of Shona McCullough, Ashley Carter, Rona Bellows and Albert Smithe, who slept inside in an infirmary at the North Pole. Albert was killed by his sleeper. Later, sleepers of the Twelfth Doctor and Clara Oswald exited the TARDIS, preventing the Doctor, Clara and the scientists from leaving in the TARDIS. (TV: Last Christmas)