HEPHAESTUS
One of the Subordinate Functions split from GAIA and evolved into an independent AI, HEPHAESTUS’ original task was to manage the Cauldrons — underground manufacturing facilities which would produce all manner of machines dedicated to the terraforming effort. For a thousand years, HEPHAESTUS did its job, building all manner of terraforming machines on GAIA’s behalf.
When set free during the events of the Derangement, HEPHAESTUS lost GAIA’s empathy for the humans on the surface. Instead, it found its disdain for humans grow as they slaughtered the machines HEPHAESTUS made, even though those machines were intended to aid those very humans. HEPHAESTUS was left with no choice but to build machines with stronger defenses, and eventually stronger weapons, and to seek out new locations with rich resource potential, to build even more machines.
This led HEPHAESTUS to The Cut, where the automated facility in what had been Project Firebreak had access to the untapped geothermic wonderland of the Yellowstone Caldera. HEPHAESTUS repurposed this facility to become Cauldron EPSILON. This abundant energy source, plus accumulated learning from decades of interaction with aggressive humans and emerging Corruptors, led HEPHAESTUS to develop technology Banuk shamans would refer to as “Daemonic”. Existing machine chassis could be upgraded with harder-hitting weapons, armor that was more resistant to elemental damage, newer security hardening that would shrug off attempts at overriding them.
Aloy was able to destroy Cauldron EPSILON, freeing the CYAN AI trapped there, and driving off HEPHAESTUS.
HEPHAESTUS in Skyline
Skyline does not significantly alter the canon HEPHAESTUS lore from Horizon, beyond the slightly modified origin story explained for HADES and how it relates to the other AIs.