Robomen

Robomen were humans turned into mindless slaves by the Daleks during their invasion of Earth in the 22nd century. They were used by the Daleks because there were relatively few daleks on Earth.

The Daleks captured prisoners and operated on their brain in an operation called the Transfer, plus fitting them with large helmet-like headsets. Robomen did not last long, eventualy breaking down and going insane, eventually becoming suicidal and dying. therefore the Daleks constantly had to replenish their numbers. (DW: The Dalek Invasion of Earth)