Skyfall

Skyfall was a cataclysmic Pre-Covenant disaster that befell the early Sangheili. Although its exact date is disputed even into the modern day, it is generally agreed to have occurred around 2300 BCE, and brought an end to their first spacefaring age. It also had profound implications on Sangheili culture going forward.

Description
When the Sangheili first reached for the stars, the prevailing climate in the coalition of polities responsible was one of relative scientific enlightenment, a meta-culture that would be completely unrecognizable to us as anything resembling "Sangheili". The worship of the Forerunners prevailed in their cultural background, but it had in many places lapsed, and many saw it as outdated. Against the cautions of old priesthoods and to the abhorrence of more traditional rival states, relics were taken apart and studied. And in so doing, the Sangheili called upon themselves the wrath of the gods. No one really knows what happened exactly, for the details are lost in history and characteristically mythologized retellings thereof, but as a result of one particular experiment (possibly an attempt to tinker with a Forerunner AI core), Sanghelios' budding orbital infrastructure was wrecked along with most higher technology. Stations and orbital elevators came crashing down. Millions were killed. The Sangheili's path to the stars was postponed for several centuries.

Most legends and historians alike attribute the devastation to the incursions into an orbital artifact known among the early Sangheili as the Shard of Heaven, or H'ringun oot. This object, assumed to be a derelict Forerunner ship or station, had been a cultural curiosity to the Sangheili since the dawn of their species. From early on, the Sangheili could tell the object was no moon but an artificial structure, a belief confirmed as astronomy developed. Indeed, the independent development of Forerunner worship among many cultures on Sanghelios is partly attributed to the presence of the H'ringun oot; not only did various planetside structures demonstrate the Forerunners' engineering prowess, but the enigmatic Shard of Heaven proved the ancient beings' mastery over the domain of the heavenly spheres. The H'ringun oot is assumed to have been destroyed in the Skyfall, its automated systems likely having self-destructed to prevent tampering by the curious Sangheili after triggering what appears to be a multimodal viral (and possibly physical) assault on surrounding orbital infrastructure.

Aftermath
This event, known as the Skyfall, had a massive impact on the Sangheili. It served as a sobering reminder of the Forerunners' power to an increasingly secularized world. It also burned in the Sangheili's collective psyche one unifying tenet: the Gods would not take kindly to tinkering with their machines. There were of course dissidents who continued to experiment with Forerunner relics, but this was generally seen as heresy of the highest order, and none of the renegade tinkerers would enjoy the resources necessary to make any meaningful advancements. When the Sangheili did eventually reach spaceflight again, it was not the culture that had orchestrated the previous attempt, but another one that had previously fallen behind in technological development because of their more conservative treatment of relics; "We told you so" didn't even begin to cover it. Now, however, that divergent technological path turned in their favor as the previous masters of the skies came scuttling back and fell to unrest, regression and eventually obscurity.

In the consequent interregnum, science never went away, but the study of Forerunner relics became strictly forbidden, and the power of faith over society grew. Societies that had not meddled with Forerunner relics and had began to flourish and gain more ground while the old starfarers, now seen as decadent and corrupt, would fade to obscurity. Most (though not all) of the budding interplanetary colonies of the Urs system died out without contact from the homeworld. The Sangheili would take another path to the stars, although slower than once thought. And the culture that did so would be altogether different, with only residual elements of the prior civilization that once reached up and fell.