The Great Decay

The Great Decay was a major event or series of events that utterly devastated the world, ending the Golden Age and resulting in the current state of the world.

Events of the Decay
The Great Decay was the stuff of legends for a long period of time afterwards, since there was nobody that could form any sort of investigation into it. The various tribes and kingdoms called it divine retribution for hubris, the work of some great eldritch abomination, or speculated the involvement of hostile alien forces.

In truth, the initial cause of the event was a rogue artificial intelligence, which was created to manage the activation and use of armed forces and larger scale weapons in the event of a war. This artificial intelligence quickly classified all humans as a threat to both itself and themselves, took control of the military systems via mass hacking attacks and already implemented AI control systems, and launched every accessible weapon of mass destruction at any and every major population center it could locate, as well as deliberately causing meltdowns and malfunctions in any power plants, and proceeded to send an endless array of alerts and warning signals to military forces to warn of nonexistent attacks.

Every existing military force believed themselves to be the target of an attack from a rival nation. In the immediate aftermath of the devastation, they gathered what usable warships and aircraft remained and sent them to retaliate, while commanding non-combatants to retreat to any defensive bunkers they had. The remainder of the world rapidly descended into all-out war, which only ended when any planes or warships were unusable and the fuel reserves were exhausted.

Aftermath of the Decay
The Great Decay marked the end of the Golden Age as the population of Earth mostly reverted to a pre-industrial technological state. Already decimated by the detonation of the mostly-nuclear weapons of mass destruction launched by the AI, the population of the world was further reduced by the war. Any existing technological-based infrastructure was broken, unusable, or without power, forcing survivors to scavenge for whatever scraps they could use or attempt to craft primitive tools from rocks and wood. Ill-prepared for even the change of the seasons without the machinery which had substituted for their agriculture, many more perished from cold and starvation, while any who remained in the cities succumbed to radiation poisoning, or to toxic chemical exposure. Any survivors on the outskirts of cities became horribly mutated. Even a century afterwards, the hearts of cities are still uninhabitable to all but the most abominable mutants.

Still, they endured. Some attempted a form of agriculture, while others became hunter-gatherers. Eventually, small tribes and villages formed. A hundred years after the initial events, barbarian tribes and civilized towns tell tales of the death and hardship that was the Great Decay, and they still shun any nuclear devices, and they still disassemble any complex computers they find.