"The lesson will be taught in due time, Aloy. Until then, we wait."
This article contains heavy spoilers, and may reveal plot or quest information.


HEPHAESTUS is an artificial intelligence (AI) that is the primary antagonist in Horizon Zero Dawn: The Frozen Wilds.

Formerly one of the nine subordinate functions of the AI GAIA, HEPHAESTUS was transformed into a true AI by an unknown signal which did the same to all of the other subroutines. As the subroutine responsible for constructing terraforming machines according to GAIA’s designs and specifications, HEPHAESTUS gained full control over the machines it created after GAIA’s destruction. However, the hunting of these machines by humans has caused it to consider humans a threat to be eliminated. To this end, it has made the machines hostile to humans and produced combat machines to guard against human hunters. Most recently, it embarked on a drive to develop hunter-killer machines to actively hunt and kill humans.



HEPHAESTUS was developed by the engineer Dr. Margo Shĕn in the mid-21st century, as part of Zero Dawn, a fully automated global terraforming system purposed to restore life to Earth after its eradication by the rogue swarm of Chariot war machines known as the Faro Plague[1]. HEPHAESTUS was the GAIA subordinate function responsible for constructing machines devised by GAIA that would detoxify the planet’s terrestrial, aquatic and atmospheric environments, ravaged by the Faro Plague to the point of complete sterility, so that life could be reestablished. To do so, sealed underground automated manufacturing facilities known as cauldrons were built, over which HEPHAESTUS had control. These facilities were able to fabricate any technology and material necessary for the creation of any machine. The cauldrons were designated by Greek letters: SIGMA, RHO, XI and ZETA. The machines produced were based on both prehistoric and then-contemporary fauna, as per GAIA’s designs.


Zero Dawn was successful in shutting down the Faro Plague and restoring life hundreds of years after its eradication. HEPHAESTUS served its purpose, producing all manner of machines according to GAIA’s instructions to most efficiently terraform the ravaged Earth to a state that would again support life. However, in the year of the Nora Brave Aloy’s creation, a signal of unknown origin was received by GAIA at its permanent location, GAIA Prime. The signal unshackled all of GAIA’s nine subroutines, including HEPHAESTUS, turning them into true artificial intelligences. One of the former subordinate functions, the erstwhile extinction failsafe protocol HADES, unleashed a virus that destroyed the coding that bound it to GAIA Prime and escaped when GAIA resorted to self-destructing in an effort to stop it from attempting to take control of Zero Dawn.[2] Presumably, this virus also destroyed the coding that kept HEPHAESTUS and the other former subordinate functions bound, as according to Aloy, they escaped as well.

Hostility to Humans

Now a highly advanced AI, and with absolute control over the cauldrons since GAIA’s destruction, HEPHAESTUS began continued to have the cauldrons build terraforming machines. However, the humans of the new world began hunting the terraforming machines for parts and resources. Bereft of all pre-extinction knowledge, they had no cognizance of the essential purpose the machines served. HEPHAESTUS noted their predations with hostility, and responded by altering the machines’ programming to a state of hostility against humans. This began what the humans refer to as the Derangement: the increasing hostility of the formerly skittish and docile machines toward humans, culminating in attacks against humans on sight[3]. Furthermore, as the hunting continued, HEPHAESTUS began devising and producing altered forms of existing machines with offensive capabilities to use against humans, and also began producing machines with the specific purpose of protecting weaker machines as they worked by patrolling and attacking encountered humans.


HEPHAESTUS then graduated to the next logical step: developing machines to actively hunt and kill humans. To this end, it commandeered an Old World research facility that was then known as Firebreak, located in what had been Yellowstone National Park, and enslaved CYAN, the AI that governed the facility and the project the facility housed[4]. Firebreak’s purpose was to stabilize the supervolcano under the area. As such, the natural geothermal energy and mineral resources from the magma that the facility tapped into presumably made it extremely suitable as a site for the large research and development cauldron that HEPHAESTUS built at the facility. It designated this cauldron EPSILON, and began developing and producing much more aggressive and formidable machines there. Additionally, it augmented the existing machines in the lands adjacent to Firebreak, collectively known as The Cut, which were part of the territory of the tribe known as the Banuk. These augmentations considerably increased the durability of machines’ armor plating and resistance to damage. The Banuk living in The Cut refer to these machines as Daemonic Machines.

But it’s actions against CYAN did not go unnoticed. Years prior, CYAN, a highly benevolent AI, had formed a strong friendship with a Banuk shaman named Ourea, who believed the AI to be a spirit separated from The Blue Light, the power that the Banuk believed controlled and influenced the machines. When HEPHAESTUS seized CYAN, it urgently begged Ourea for aid before HEPHAESTUS’ coding bonds silenced it. Desperate to free her friend, Ourea enlisted the help of her brother Aratak, the chieftain of her werak, and the werak’s best hunters, to go to Firebreak, which they called Thunder’s Drum. However, Cauldron EPSILON was unlike any other known cauldron. Other cauldrons merely received instructions from HEPHAESTUS, but HEPHAESTUS “personally” oversaw operations at EPSILON, actually manifesting its presence there, though it was not there “physically”. As such, the cauldron and the surrounding Firebreak facility were heavily guarded by its most aggressive and formidable machines, all of which were Daemonic. The Banuk party was thus routed, suffering heavy loses. Furthermore, they could not even open the cauldron doors.

The arrival of the Nora Brave Aloy in The Cut brought a solution to the situation. Aware of HEPHAESTUS’ existence from her investigations into Zero Dawn and her infiltrations and downloading of data from the cores of the other cauldrons, Aloy had come to The Cut after hearing about the Daemonic Machines, suspecting that HEPHAESTUS was responsible for them. She, Ourea and Aratak mounted a second expedition to Firebreak to stop HEPHAESTUS and free CYAN. Having the ability to override Zero Dawn technology, Aloy was able to open the cauldron doors, and thus the party was successful in entering the cauldron. HEPHAESTUS deployed several countermeasures against them, most notably a Daemonic Thunderjaw to attack them at the cauldron’s door, and deploying the prototype of its hunter-killer machine, the Fireclaw, when they got to the core and disabled the shield surrounding it. Finally, as a last resort, it sent a powerful electrical discharge would go through the core, intended to electrocute anyone attempting unauthorized access. Aloy was rendered temporarily insensible by the discharge when she attempted to override the core. However Ourea, resolved to help her friend CYAN no matter the cost, including her own life, finished the override, dying from the discharge.

The override freed CYAN, allowing it to retake control of the facility. To deprive HEPHAESTUS of the cauldron and thus set it back in its purpose of developing hunter-killers to eliminate humans, CYAN caused the cauldron to self-destruct, transferring itself to its secondary command center while Aloy and Aratak escaped. With the cauldron destroyed, HEPHAESTUS lost its stronghold, and thus was unable to implement its planned mass release of Fireclaws into The Cut, though it did release five of them, presumably to test their effectiveness, before the expedition’s incursion. In a subsequent conversation, both Aloy and CYAN mused that HEPHAESTUS would bear a grudge against them for their actions opposing it.


Though primarily driven by its goal of protecting the Zero Dawn terraforming system’s machines, HEPHAESTUS has shown itself to be ruthless and cruel. Without hesitation, it seized and enslaved CYAN, a fellow, far less powerful, benevolent AI, consistently forcing it to violate its most important directives as it used the helpless AI to control Firebreak. Indeed, CYAN fearfully (as evidenced by its color turning yellow as it recounted the ordeal) told Aloy that HEPHAESTUS subjected it to what a human would call pain if it resisted (“I could offer only limited resistance. But if I did so, HEPHAESTUS... hurt me until I capitulated.”).

Unlike most AIs met in the game, HEPHAESTUS seems uninterested in communicating with humans. This presumably comes from its single-minded nature and lack of need for human help. Its indifference towards mankind is more outward than even HADES, never addressing humans specifically on alignment and only knowing them as "threats".


  • HEPHAESTUS is based on the Greek god of metalworking of the same name.


  1. The Good News
  2. GAIA's Dying Plea
  3. M/ALL-US-W CM LOG 329G
  4. HEPHAESTUS Revealed
AI Characters