Toclafane

The Toclafane were the final evolution of the Human race, cyborgs integrated into a spherical, mechanical shell. The name "Toclafane" was given to them by the Master, after a fairy tale monster from Gallifrey, the local equivalent of Earth's "Boogeyman".

Biology
The Toclafane were cyborgs, the heads of Humans integrated into metal spheres that were sealed shut, making themselves "pretty". 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.

The spheres could hover through the atmosphere and even fly through space, undamaged. Concealed within the sphere were spikes and blades that could be extended. A Toclafane could use them to slice up a Human being. The spheres were also equipped with a laser that can turn Humans into ash. The spheres could be shot down by an electrical surge of 58.5 kiloamperes, transferred charge 510 megajoules precisely; however, this was most unlikely because this power existed in lightning bolts, and the odds of one bolt striking a Toclafane were extremely unlikely. (DW: 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", and being that the Utopia Project was set up to find a way to survive the end of the universe, everyone believed they found a way. Humans attempted to travel to the mythical paradise, but what they found was a dark, cold, and inhospitable world. To survive, the Human race "evolved" into the Toclafane.

The Master, using the Doctor's TARDIS, travelled to the planet in the year 100,000,000,000,000 where he met them. As Prime Minister of Great Britain, the Master announced that 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 past history. Thanks to the machine, the universe did not intervene to alter the paradox of Humanity being subjugated by their own descendents. 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. They subjugated Humanity and forced them to build a fleet of 200,000 rockets to take over the rest of the Universe. The Toclafane were able to kill their ancestors without cancelling themselves out due to the paradox machine keeping the paradox in place. Their plans were thwarted one year after the invasion by Jack Harkness, who destroyed the paradox machine, forcing the Toclafane back to the end of the universe on Utopia, without Humanity even remembering that they invaded at all. With the Master's subsequent death preventing the Toclafane from being brought back in time, they were trapped in their future time zone and presumably died out alongside their cousins on Malcassairo, which ultimately brought Humanity to extinction. (DW: The Sound of Drums / Last of the Time Lords)

Behind the scenes

 * The appearance of the Toclafane's laser weapon discharge is similar to that of the Judoon blaster.
 * The voices of the Toclafanes 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 DW: 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 REF: Doctor Who: The Writer's Tale - The Final Chapter.