Toclafane

The Toclafane were the final form of the human race, cyborgs integrated into spherical, mechanical shells. The name "Toclafane" was given to them by, after a fairy tale monster from Gallifrey, the local equivalent of Earth's "Boogeyman." (TV: The Sound of Drums)

Biology
The Toclafane were cyborgs used as the Master's brawn and muscle when taking over Earth. They were the heads of humans from the year 100 trillion integrated into a dark-grey sphere-shaped metallic shell, held together with a magnetic clamp. The humans heads have wiring plugged into their wizened heads. The spheres were very little over a foot tall and were sealed shut. They became a hive mind, allowing them to see the destruction of an individual as no threat. Their personality was amoral and childlike from a failed attempt to regress themselves to children; instead having rather primitive emotional traits and an enjoyment in killing their great ancestors because "it's fun".

Each Toclafane had built-in life-support and teleportation systems. The shells were capable of flight, and could survive the harsh climate and temperature of space with no side-effects. Concealed within the shell were extendable razor-sharp blades and spikes, used as their up-close-and-personal weaponry. The spheres were also equipped with red laser guns that could disintegrate life forms. The spheres could be shot down by an electrical surge of 58.5 kiloamperes, with a transferred charge of 510 megajoules precisely. Man-made electrical barriers could be constructed to sustain the right energy level balance needed to successfully fry the technology incorporated into the Toclafane spheres, rendering them useless and making them drop to the ground, although the human head would still survive. (TV: The Sound of Drums/Last of the Time Lords)

History
The last of humanity, scattered across the darkness of space, received a message that said "Come to Utopia". Because the Utopia Project was set up to find a way to survive the end of the universe, everyone believed they had found a way. Humans attempted to travel to the mythical paradise, (TV: Utopia) but what they found was a dark, cold and inhospitable world. To survive, the human race "evolved" into the Toclafane, essentially cannibalising themselves and regressing to children. Even so this was not enough, as the universe was still collapsing around them.

, using the Doctor's TARDIS, travelled to the planet in the year 100,000,000,000,000 where he met them. Over time, he became master over them and devised a plan to enable them to change history and survive. He soon saw them as his "children". (TV: Last of the Timelords)

As Prime Minister of Great Britain, the Master announced he had made contact with a race of friendly aliens. The Toclafane had tractable, child-like personalities and trusted the Master. The Master had, by this time, converted the TARDIS into a paradox machine, allowing the six billion Toclafane to invade Earth in their own past and change their own history. With the Toclafane, the Master built the beginnings of a new Time Lord Empire with the army of six billion Toclafane. The Toclafane invasion was a success. The Master ordered the Toclafane to remove one tenth of the population of Earth. They subjugated humanity and forced them to build a fleet of 200,000 rockets to take over the rest of the universe. Their plans were thwarted one year after the invasion by the Doctor and his associates. Jack Harkness destroyed the paradox machine with an assault rifle, forcing the Toclafane back into their relevant time-line. Humanity, aside from those on the Valiant, did not remember they had been invaded.

Trapped at the end of the universe the Toclafane's fate remains unknown, but they presumably either died with Utopia, or escaped the planet into the now empty void of their dead universe. (TV: The Sound of Drums/Last of the Time Lords)

Behind the scenes

 * The voices of the Toclafane sound the same as those of the Gelth, due to the use of a team of regular voice artists for aliens (except those voiced by a celebrity guest), many of whom voiced both the Gelth and the Toclafane.
 * Russell T Davies came up with the original design for the Toclafane when he was coming up with a potential alternate creature to use in the episode Dalek, had rights issues to the Daleks not been worked out. This early version was called "Future Human" and Davies' original illustration is included in Doctor Who: The Writer's Tale - The Final Chapter.