Toclafane

The Toclafane (bogeyman in the Time Lord language) of the Doctor Who Universe are the last descendants of humanity, living around the year 100 trillion and wandering across space just as the ‘cold death’ of the Universe began. They are notable for their last-ditch effort at survival: the invasion of Earth, circa 21st century.

History
In the year 100 trillion, the last remnants of Humanity were living in the planet Malcassairo. It is notable that they have reverted to their 21st century form. There they lived in large underground colonies, similar to ant colonies. Due to the “death of all the stars”, Malcassairo was in eternal night, with the colonies being the only refuge from both the cold and the Futurekind, a vicious, carnivorous race roaming the Malcassairo plains for human prey, yet is itself implied to be another human descendant.

In one level of the colony, humans placed their hopes on Dr. Yana, an old scientist, to find a way to leave this last refuge and go to the mythical place of Utopia, a planet known to have been safe from the impending doom. His efforts has led to the creation of a large spacecraft, resembling a giant rocket. Due to the arrival and help of the Doctor, the rocket was made active.

It turned out Dr. Yana was in fact The Master, a renegade Time Lord. He escaped on the Doctor's TARDIS, leaving him and his companions as the Futurekind attacked the base on Malcassairo.

The Master took the Toclafane to the alleged location of 'Utopia', but it was not what they had wished for. The Master showed them to put their heads in the metal cases which became the Toclafane.

The Master used the TARDIS to create a paradox machine, which enabled the Toclafane in 21st-century Earth to murder their ancestors without affecting their own existence. They subdued the population and forced them to build a fleet of rockets, to take the Toclafane to the stars in the beginnings of a new human empire.

However, the Doctor and his companions defeated the Master and destroyed the Paradox Machine, trapping the Toclafane at the end of the Universe and "undo-ing" the damage they have inflicted.

Biology
For all intents and purposes, the Toclafane are cyborgs. From outside, they assume the appearance of a segmented, stainless steel ball with the ability to extend weaponry: from sharp spinning blades to the more modern laser.

Inside, however, is the “real” creature: a wrinkled, hairless human head, perhaps indicative of the stresses of time and space travel or perhaps simple aging. The creature is completely assimilated with the machine, and is thus completely dependent on it for communication, mobility, and nutrition. The shell also enables the creatures to breathe and travel in outer space, presumably protecting it against radiation, projectiles, and the vacuum. It is shown that they have advanced navigational and flying capabilities.

There is little evidence that they could use their traditional senses, as their cyborg body includes a camera and a speaker which they use to communicate with Earth humans and each other.

So while their biological “body” is basically Homo sapiens and are quite vulnerable, the metallic shell protecting it is very tough. So tough, in fact, that only an electrical surge of 58.5 kiloamperes, transferred charge 510 megajoules precisely can bring one down. Conventional human weaponry such as bullets and rockets are ineffective against inflicting damage on the shell. During their one-year occupation of Earth, one was brought down by a lightning strike in South Africa. Martha Jones and surviving members of the Earth resistance then attempted to duplicate this, with success.

Toclafane psychology is a topic by itself. While technologically, they are superior to their 21st Century ancestors, they possess the emotional maturity of children. They are also quite cruel, using their lasers and blades quite liberally during their occupation of Earth and enjoying the sensation of killing their ancestors with a high-pitch, child-like laugh. Their limited emotional development has limited their speech into simple sentences.