The Reapers' origins began more than one billion years before the events of the "Mass Effect" series, when the Milky Way galaxy were controlled by a species called the . Observing a repeating cycle in which their contemporary civilizations would collapse after creating synthetic lifeforms that turn on their creators, the Leviathans seek to break this cycle by, ironically, creating a synthetic intelligence called the Catalyst. The Catalyst was intended to bridge the differences between organic and artificial life, and to preserve organic species at any cost. The Catalyst eventually came to a conclusion that conflict is inevitable, and decided to achieve this by harvesting organic civilizations at their peak, shortly before they are responsible for their own downfalls, and absorbing their genetic material into new lifeforms that it can maintain and preserve forever. The Catalyst begins with its own creators, the Leviathans, transforming them into the first Reapers and beginning the war its creators had tried to avoid. Their origins inform the overarching design of the Reapers, with the implication that they are thinking creatures which have adopted patterns of living, organic beings in their construction. As part of their perpetual plan for the galaxy's organic species, the Reapers set up the Citadel and created the docile Keepers as its caretakers; it is in fact a giant mass relay, the mass transit devices ships used to traverse the galaxy. The Reapers allow organic life to develop, with the Citadel serving its purpose as a hub for the intergalactic species of the Milky Way galaxy, for 50,000 years before they come through the Citadel from dark space to harvest it. The Reapers mind control their victims through a process known as indoctrination; each Reaper possesses some kind of mechanism which subtly influences the minds of any organic individual in range over time, gradually rendering them incapable of independent thought. The Reapers sometimes alter their servants into synthetic-organic shadows of their former selves; the Collectors are an example of a race of creatures who have been genetically mutated and extensively altered with cybernetics from Prothean victims long before the events of the "Mass Effect" trilogy. Besides "husk" units, mindless synthentic-organic creatures who were converted from harvested human bodies impaled on spike-like structures called "dragon's teeth", Reaper ground forces in "Mass Effect 3" mainly consist of horrific, twisted versions of the galaxy's various species, including asari, batarians, krogan, turians, and the rachni, a sentient race of telepathic insectoid beings long thought to be extinct.