A decent way into Horizon Forbidden West, Aloy is given a mission which unfurls on multiple fronts — tracking down three of GAIA's missing subordinate functions. These three are AETHER, POSEIDON, and DEMETER, and each is given its own main quest. The game presents these as though the player has a choice in the order in which they are sought out, but that's not actually the case, and players may find they are unable to progress if they choose the wrong one first.
In Horizon Forbidden West, every subordinate function's purpose contributes to GAIA's ability to rebuild the Earth's biosphere after it collapsed in the 21st century. GAIA is the AI overseer of Project Zero Dawn, a program established to bring life back to the planet after it is snuffed out by a swarm of self-sufficient robots known as the Faro Plague, which swept across the globe consuming all biomass. Each of GAIA's subordinate functions are responsible for different aspects of Project Zero Dawn, but were upgraded to independent AI by a mysterious signal that was received prior to the first game.
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Relatively early on in Horizon Forbidden West, Aloy has secured a backup of GAIA, and has already brought the MINERVA subroutine back into the fold. Additionally, GAIA has deduced that the rogue signal which converted the subordinate functions to AIs was an act committed by Horizon Forbidden West's mysterious Far Zenith, another 21st century organization hoping to create a new future for humanity. GAIA then informs Aloy that three more subordinate functions are within reach: AETHER, responsible for detoxifying the atmosphere; POSEIDON, in charge of reconstituting the oceans and marine life; and
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