Decades after the initial conflict ended in a truce, the Cylons reappear and launch a cataclysmic attack on human civilization that kills billions.

The metallic Centurions are secretly led by several models of synthetic humanoid Cylons who are virtually indistinguishable from humans and have infiltrated their society.

In the original 1978 series Battlestar Galactica, created by Glen A. Larson,[1] the Cylons are a race of sentient robots at war with the Twelve Colonies of humanity.

[16] Characters within the series either experience or discover instances where naturally evolving humans developed artificial intelligence, only for the newly sentient life forms to wage catastrophic wars against their creators.

[19] The biological models have (at times) demonstrated enhanced strength, stamina, and an ability to interface with computer systems; however, they also feel pain, hunger, and fear.

[15][22] All Cylons in the re-imagined series use a digital consciousness employed in mechanical or biological media; the primary capability this allows is the ability to "download" or "resurrect" into a new body.

[25] As a byproduct of digital consciousness, Cylons have an ability referred to as "projection", a form of realistic daydreaming that allows them to change the appearance of their surroundings in their own mind (e.g. making the otherwise identical corridors of a ship look like a forest, etc.).

Philosophically, the Cylons recognize that they are the metaphorical "children" of humanity; as such, they envisioned the design of their biological models as a natural evolution in becoming closer to their creators and as a way of eventually surpassing them.

When the biological Cylons determined that they could not reproduce sexually, the more dogmatic faction experimented with a forced breeding program on captured human women, with no success.

"[32] About 6,000 years before the events of the re-imagined series, the naturally evolved humans of Kobol existed in twelve tribes with advanced technology, eventually developing self-aware machines that rebelled and waged a devastating war.

[19] About 2,000 years before the events of the re-imagined series, the descendants of the Thirteenth Tribe created their own race of self-aware machines for subservient labor and were, in turn, killed by their creations in a nuclear war.

[citation needed] Nearing the end of the conflict, the Cylons began experimenting with their first attempts to evolve with biological components, using captured humans as resources and creating their prototypes for the next generation of basestars and hybrids.

[15] After forty years, the Colonial Admiralty sent Battlestar Valkyrie on a covert mission close to the armistice line to gather intelligence on Cylon activity.

Ten months after the initial attacks, Eight Sharon Valerii (call sign "Boomer"), attempted to assassinate Commander Adama under the influence of programming unknown to her.

Fearing that the Cylons might capture the child, President Laura Roslin faked Hera's death and secretly had her adopted by a human woman.

Outnumbered and outgunned, the Galactica, Pegasus, and the rest of the colonial fleet in orbit fled, leaving the Cylons to occupy the human settlement unchallenged.

After the inhibitor modules that restrict higher functions, granting them independent thought were removed, although still largely servile, the Centurions were shown to exhibit human-like behaviour, such as politeness and consideration (e.g., responding to "please").

Modern Raiders, unlike their predecessors in the First Cylon War, are alive, with a complex system of veins, organs and biological fluids contained within their main body.

Raiders, by revealing their red "eye", can transmit deadly computer viruses or signals to enemy ships, which strike with brutal efficiency and can shut down Colonial vessels, turning off their power and making them easy prey.

Humanoid Cylons are indistinguishable from humans due to their creation through synthetic biology, employing real bones, flesh and blood rather than metal, but still possess a "digital molecular structure".

Prior to the "Final Five" arriving to stop the first Cylon War on Caprica, the Centurions had already made their first humanoid, called the Hybrid, for the purpose of controlling the baseship.

When Centurions on one of the Cylon baseships received this modification and thus developed free will, they revolted against Cavil because they rejected his policy of removing the biological brains of the Raider ships and returning them to a purely mechanical form so that they would follow orders without emotion interfering.

Because of their common Kobolian ancestry, this "thirteenth tribe" kept their own records and religious history, which shared some of the prophetic events outlined in Colonial scriptures; in particular, the departing thirteenth tribe created the "Temple of [the] Five" on their way to "Earth", and the Final Five returned to it on their way back, with Colonial texts predicting its creation and third visitation by the Humans and Cylons within the series.

Their purpose, and how and why they were hidden from the rest of their kind, is a major plot point of the fourth and final season when Ellen Tigh is revealed to be the fifth member.

A standoff between Galactica and the rebel Baseship exposes the four as Cylons to the humans, but Lee Adama, in his capacity as acting President, grants them amnesty as part of an alliance ("Revelations").

In a rage, he strangles her, shattering the temporary peace, although a fortuitous nuclear explosion and Cavil's subsequent suicide effectively ends the war moments later.

After he is shot in the head and enters a vegetative state, Sam Anders is connected to a datastream and as a result develops Hybrid-like abilities, whereupon he acts as Galactica's Hybrid.

This monotheism seems to share some of the characteristics as the Abrahamic religions: belief that God is omniscient, omnipotent, omnibenevolent, that he will one day deliver divine retribution and that he intervenes in the mundane world.

A Six engaged Kara Thrace hand-to-hand and proved to be exceptionally fast, agile and strong ("Kobol's Last Gleaming II").

Presented in The Gun on Ice Planet Zero book, the second of the two novels written before the network insisted the Cylons become robots, their multibrain status is the surgical inclusion of an auxiliary brain, allowing for higher-level thinking abilities.