After Ted Faro founded his company, he decided to change from environmentally-friendly machines to military projects for later use. This move was met with much criticism from the press and fellow scientists, among them Elisabet Sobeck. However, Faro pressed on and soon made largely self-replicating machines that had the capacity to take in biomass as fuel.
Soon after, however, the Chariots were hit with a glitch, severing the chain-of-command from the company, and they began to self-replicate at an alarming rate, using biomass as their primary form of fuel. This glitch led to the realization that the world would soon be stripped of biomass. In response, humans began to work on Project Zero Dawn as a life-saving work and Operation: Enduring Victory as a means to stall the Faro Plague.
After nearly two years of fighting, the human population was extinct, and eventually the machines were rendered dormant by lack of biomass.Once GAIA cracked the codes to activate the deactivation, the machines were rendered useless. They were then mostly buried under the dirt and largely forgotten. That is until the Eclipse began to unearth them by order of HADES.
Events of Horizon Zero Dawn
Due the instigation of HADES, the Eclipse soon began to rediscover the machines and attempted to amass control of them, but their mission was largely a means for HADES to arrive at The Spire to send a signal to reactivate the Plague, beginning it once more. Though it worked momentarily, Aloy managed to defeat HADES and rendered the Chariot line useless once again.
The Chariot line was made up of three broad classes. These were primarily meant for Peacekeeping or military purposes until the Plague turned them against all of humanity.
Horus-class machines (also called Metal Devils) were the largest of the Chariot line and were used for large scale attacks. They were also the primary location for the replication of some of the other Chariot Line machines. Crab-like in appearance, they used massive arms and multi-function "eyes" to attack targets below. They were rendered useless long before Aloy's time.
Kopesh-class machines (also called Deathbringers) were the second largest of the Chariot line and were used for full scale frontal assaults. These particular type of machines had multiple gun ports or missile weapons. However, they were prone to overheating from overuse.
Scarab-class machines (also called Corruptors) were the smallest and most agile of the Chariot line. Though small compared to the other lines, they carried devices that fired missiles or corruption signals and a gunport. They could jump great heights while also reaching far distances. Should one try to escape, the machine used a long tail to attack targets. However, they were vulnerable to fire damage.