Toclafane

The Toclafane were Human-based cyborgs, integrated into a spherical, mechanical shell. The name Toclafane was given to them by the Master, after a fairy tale monster from Gallifrey; their name, like the Bogeymen on Earth, was just made up.

Biology
The Toclafane were cyborgs, the faces of Humans integrated into metal spheres that are sealed shut, making themselves "pretty". They became a hive mind, making themselves almost invincible, as well as evil and child-like in their personality. They are childlike because they tried regressing themselves into children, but failed.

The spheres could hover through the atmosphere and even fly through space, undamaged. Concealed within the sphere are spikes and blades that can extend. A Toclafane can use them to slice up a Human being. The spheres are also equipped with a laser that can turn Humans into ash. The spheres can be shot down by an electrical surge of 58.5 kiloamperes, transferred charge 510 mega joules precisely; however, this is most unlikely because this power exists in lightning bolts, and the odds of one bolt striking a Toclafane are slim.

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, their final evolution.

The Master, using the Doctor's TARDIS, traveled 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. 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 without knowing they even invaded. 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, 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 weapons are the same as the effect 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.