Horizon Forbidden West can be a very long game, and depending on how much side content players decide to engage with, it may be a while until they are given one of the game's larger objectives - finding GAIA's subordinate functions. At the beginning of the game, Aloy is looking for a back-up of GAIA, the AI created to oversee Project Zero Dawn. She fails to find one in Horizon Forbidden West's Far Zenith facility, but soon locates a back-up and is subsequently tasked with retrieving the subordinate functions.

Project Zero Dawn began in the 21st century on the eve of an apocalypse caused by a rogue machine swarm which used Earth's various biomasses for fuel. The war machines, collectively referred to as the Faro Plague, were unstoppable because of their self-sufficiency and a sophisticated encryption installed by its creator, Ted Faro. Roboticist and engineer Elisabet Sobeck guilted Faro into bankrolling Project Zero Dawn, a massive undertaking that would rebuild the Earth's biosphere after the encryption could be cracked, which would be accomplished years after all life on the planet had been snuffed out.

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Sobeck was responsible for the creation and training of GAIA, an artificial intelligence that commanded nine subordinate functions, each with a different purpose. When Aloy successfully retrieves a backup of GAIA in Forbidden West, the AI is unable to reverse the damage being caused to the environment. The backup Aloy obtained consisted of GAIA only, and the AI is only marginally useful without its entire array of programs. Thus the need for Aloy (and the player) to seek out the subordinate functions in the Forbidden West.

Why GAIA Lost Her Subordinate Functions

Horizon Forbidden West subtly pushes the player to find the three subordinate functions in the correct order

Years prior to the events of Horizon Zero Dawn, just before Aloy's birth, a mysterious signal was received by GAIA that turned her subordinate functions into AIs themselves. The subordinate-function-turned-AI HADES, which was responsible for restarting Project Zero Dawn in the event of a new biosphere being unsustainable, took the opportunity to seize GAIA's terraforming apparatus in an attempt to destroy all life on Earth once again. To stop HADES, GAIA self-destructed, with her last act being the creation of an Elisabet Sobeck clone, which would grow up to become Aloy. Piecing together this history and stopping HADES' next plot is the major thrust of Horizon Zero Dawn's story and ending.

With GAIA gone, and the subordinate functions now in possession of their own free will, they scattered to unknown locations. Perhaps the most dangerous consequence of this, alongside HADES' genocidal machinations, is a phenomenon known as the Derangement. HEPHAESTUS, the subordinate function responsible for building GAIA's terraforming machines, began to perceive the tribal societies of the 31st century as a threat since they had taken to hunting the machines. Now in control of itself, HEPHAESTUS began making machines to fight back, with their sudden hostility becoming known as the Derangement by the primitive humans. In Horizon Forbidden West, Aloy is locating the scattered subordinate functions in order to bring them back into the fold and reverse the damage being done to the biosphere.

Next: Horizon Forbidden West: All New & Returning Tribes