"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 a rogue artificial intelligence. It is the primary antagonist of Horizon Zero Dawn: The Frozen Wilds.

Formerly one of the nine subordinate functions of the artificial intelligence GAIA, HEPHAESTUS was transformed into an AI by an unknown signal which did the same to all of the other subordinate functions. As the subordinate function 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 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 created by the robotics 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 re-established. To do so, sealed underground automated manufacturing facilities known as cauldrons were built. HEPHAESTUS built the machines that GAIA devised at these facilities, under GAIA's overall control. Cauldrons are able to fabricate any technology and material necessary for the creation of any machine. Five of these original cauldrons are known, designated by Greek letters: PSI, 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. Then on August 26th 3020 at 8.45 am, Old Ones timekeeping, a signal of unknown origin was received by GAIA at its permanent location, GAIA Prime.[2] The signal turned all of GAIA’s subordinate functions, including HEPHAESTUS into artificial intelligences themselves. 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.[3] This virus also destroyed the coding that kept HEPHAESTUS and the other former subordinate functions bound, and they escaped as well.

Hostility to Humans

"So HEPHAESTUS is the source of the Cauldron data intrusions...? It’s been forcing Cauldrons to make aggressive machines… deadlier and deadlier, year by year."

Now a highly advanced, powerful AI, HEPHAESTUS attempted to take control of the cauldrons. It succeeded in doing so with the five known Zero Dawn cauldrons. However, others continue to repel it due to their robust cyber-defenses. Initially, it merely had the cauldrons it gained control over continue to build terraforming machines. However, the humans of the new world had been hunting the terraforming machines for parts and resources for centuries. Bereft of all pre-extinction knowledge, they had no understanding of the essential purpose the machines served. HEPHAESTUS therefore deemed the human species a threat to the new biosphere the machines maintained. It altered the machines’ programming to a state of hostility against humans, attempting to cull the human population.

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[4]. Furthermore, as the hunting continued, HEPHAESTUS began altering existing machines to have 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.


Approximately fourteen years after becoming an A.I., HEPAHESTUS became aware of the existence of an ancient research facility that was then known as Firebreak, located in what had been Yellowstone National Park. Firebreak was created to stabilize the supervolcano under the area. As such, the virtually unlimited natural geothermal energy and mineral resources from the magma that facility tapped into made it extremely suitable as a site for the research and development cauldron. HEPHAESTUS intended to build to develop machines to be deployed as human hunter-killers. It later enslaved CYAN, the AI governing the facility, via a malware daemon, and proceeded to create its R&D cauldron, Cauldron EPSILON.[5] Here, HEPHAESTUS began developing much more powerful and dangerous machines. Additionally, it created a means of augmenting the existing machines in the lands adjacent to Firebreak, collectively known as The Cut. These augmentations considerably increased the durability of machines’ armor plating and resistance to damage. The Banuk, the human tribe native to the Cut and the lands north of it, labelled these machines as Daemonic Machines.

However, its 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. 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. But 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 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 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. She managed to complete the override at the cost of her life.

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 auxiliary control 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. Currently, the whereabouts of the AI in question is unknown.


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 captured 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, recounting the ordeal thusly: “I could offer only limited resistance. But if I did so, HEPHAESTUS... hurt me until I capitulated.”

Though it considers humans a threat to be eliminated, HEPHAESTUS never interracts with humans directly. This presumably comes from its single-minded nature and lack of need for human help. Unlike HADES, it has never needed to interact with humans for the pursuit of its objectives; humans interact only with the machines it creates. Indeed, during its efforts to eliminate the expedition that attempted to access EPSILON’s core, HEPHAESTUS never addressed Aloy, Aratak or Ourea. Instead it noted their intrusion and deployed its countermeasures against them. In subsequent conversations with Aloy, CYAN noted that even if a single human or even an entire tribe stopped hunting machines, it would be meaningless to HEPHAESTUS, and concluded that rebuilding and rebooting GAIA is the only effective solution to regain control of it.


  • Virtual Creativity: HEPHAESTUS was designed to create any technology necessary for running and maintaining the terraforming system. Rather than limit GAIA to 21st-century designs, HEPHAESTUS was programmed to be capable of learning and developing creative solutions to problems.
  • Vocal Communication: HEPHAESTUS is capable of speech, albeit in a highly artificial-sounding voice, possibly having learned to do so through its takeover of CYAN's hardware. Unlike HADES, HEPHAESTUS' speech appears to be nothing more than the diction of its processes; it never communicates with humans.
  • Remote Manifestation: HEPHAESTUS is capable of manifesting itself from a remote location. In this way, it was able to enslave CYAN and construct Cauldron EPSILON without having to be present within Firebreak. Additionally, it was able to keep its location hidden from CYAN.


  • HEPHAESTUS is named after the Greek god of metalworking.
  • If Aloy completes Cauldron EPSILON and talks to Cyan about HEPHAESTUS before completing Deep Secrets of the Earth, CYAN informs Aloy that HEPHAESTUS is not in control of all cauldrons. Only if Aloy completes Cauldron EPSILON before Deep Secrets does CYAN provide this information.


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