Cybermen

"Cybermen will remove fear. Cybermen will remove sex and class and colour and creed. You will become identical, you will become like us."

- One of the Cybermen.

The Cybermen are a race of hostile humanoid cyborgs from Doctor Who that are one of the series' most recurring and dangerous antagonists (rivaling the Daleks in that regard).

Personality
Since Cybermen are living brains inside a robot body that robs them of emotion they don't have any free will or feelings. They are programmed to see themselves as the ideal lifeform and work to "upgrade" others into an essentially immortal mechanical body by force if needed.

Cybermen from Mondas
The first Cybermen were originally the people of the planet Mondas who converted themselves to survive when their world was knocked out of solar orbit and drifted into deep space. Having installed a drive propulsion system so they could pilot the planet itself, the Cybermen began conquering planets to convert the native life forms into more Cybermen to increase their numbers. The Cybermen first confronted the Second Doctor in the distant future when he came to one of their tombs five centuries after Mondas's destruction in 1986.

The Cybermens' first meeting with the Doctor was in 1970 when a group of Cybermen from "Planet 14" use industrialist Tobias Vaughn to paving the way for a ground invasion. However, the Cybermen are forced to retreat due to interfere from Doctor with the aid of the newly formed United Nations Intelligence Taskforce. However, an advance force of Cybermen set up base in an Antarctic space tracking station in the year 1986 to summon Mondas. However, Mondas's overloaded from draining Earth's energy for the Cybermen's use and ceased to be along with many of the Cybermen. However, the Cybermen lingered on as species and confronted the Doctor in several time periods with him thwarting them.

In the Eleventh Doctor episode "Nightmare in Silver" (2013), thought to have been extinct, the Cybermens' continual upgrade made them invincible to the point that only a planetary destruction device can be used against a single Cyberman if not dealt with quickly. These enhancements include Cyber-conversion by advanced miniscule Cybermats known as Cybermites and able to convert non humanoid life forms.

Human 2.0
Ending up in a parallel universe with his companion Rose Tyler and her boyfriend Mickey Smith, the Tenth Doctor finds the parallel Earth to be the genesis of that reality's Cybermen. The product of a dying transhumanist named John Lumic, these Cybermen were created from the homeless and acted on his will to convert every human into a Cyberman. However, the Doctor and his friends stop the Cybermen by deactivating the emotional inhibitors that keep the Cybermen from going insane. Afterwards, a resistance group, the Preachers, set about to clean up the remainder of Lumic's factories around the world.

However, thanks to the Cult of Skaro, the Cybermen exploit a breach between universes to invade London. Though the Doctor manages to send the parallel Cybermen into the void, traces of their actions in Canary Wharf remain such as a partially converted Torchwood employee named Lisa Hallett who was killed moths later by her follow Torchwood operatives.

However, due to the ctions of the New Dalek Empire, the parallel Cybermen escaped the void and ended up in London 1851, where they enlisted the aid of a human named Mercy Hartigan before they are destroyed for good by the Doctor.