Great Purification

"Activation is murder. A genocide larger than [this galaxy] has ever known. We are sworn to protect life, not destroy it! That is the Mantle we were given to carry."

- The IsoDidact, protesting the firing of the rings.

The Great Purification was the culminating act of the Forerunner-Flood war, when the seven rings of the Halo Array were fired. The rings' combined pulses scoured the entire Milky Way galaxy, disabling the Flood and wiping out all vertebrate life that was not safely outside its range at the Ark. The effects of the firing had lasting effects on the galaxy for the next 100 millennia, marking the end of the Forerunner ecumene and eventually leading to the rise of the Covenant.

Background
"The Flood cover more of our galaxy with each passing day. They feast on the essence of life itself. The only way to stop their advance is to remove that life upon which they feast."

- The Master Builder discussing the necessity of the Halo Array with the Librarian.

The Great Purification was not the first time a Halo had been fired. A ring of the original array had been fired by the Master Builder at Charum Hakkor to test its effect on the native wildlife. Later a pulse was fired on Janjur Qom to punish the San'Shyuum there who rebelled against the Forerunners. At the Battle of the Capital Mendicant Bias took control of five of the nearby rings (which had been assembled for decommissioning) and fired them all. The last use before the combined wave from the Array was at the Battle of the greater Ark, when Omega Halo was fired, damaging a Precursor star road.

Effects
"This installation has a successful utilization record of 1.2 trillion simulated and one actual. It is ready to fire on demand."

- 2401 Penitent Tangent

The biological effects of the rings had long term effects on the galaxy's ecosystems. Because the Librarian could not rescue every species, all of the specimens left behind were wiped out and rendered their species extinct. Those species that had been saved on the Ark were reseeded afterward, but the gaps in the food chain were significant enough that during the dark time many more species went extinct due to their permanently altered environment. While the Conservation Measure did its best to eliminate any trace of genetic disruption, scars still remained in the fossil record. In 2332, scientists discovered a curious anomaly dated to Late Pleistocene, in which no fossils dating to roughly 97,000 BCE were discovered on colonized by humans. The Ross-Ziegler Blip, as it was called, was initially dismissed as a random aberration caused by spatial distortion, out of doubt that an interstellar extinction event could have occurred simultaneously on every planet. After the Halos were discovered by humanity, the Blip was reinvestigated, its cause now identified as the destruction of all bio-mass in the galaxy during the Great Purification.

An unexpected effect of the Halos' firing was that Precursor artifacts were destroyed as well. This was due to the Halos interacting with neural physics, which held Precursor constructs together and made them nearly unbreakable. Firing a Halo in the vicinity of the artifacts would disrupt their atomic binds and thus disintegrate the relics. While initially viewed as an unfortunate side effect, as when a ring was tested at Charum Hakkor, this disruption turned into a small advantage for the Forerunners in their war against the Flood, letting them use the Halos to destroy Precursor star roads that were being controlled by the parasite. When the Great Purification occurred, every Precursor artifact in the Milky Way was destroyed, leaving no trace of them for modern civilizations. Shortly before the blast, the Gravemind revealed that the Domain, an immaterial dimension used by the Forerunners as a repository of knowledge, itself was a Precursor creation and so would be wiped out with the Flood. As such, when the rings fired the blast erased 100 billion years worth of stored knowledge and recorded history.

Legacy
"Halo! Its divine wind will rush through the stars, propelling all who are worthy along the path to salvation."

- The Prophet of Mercy

When the reseeded civilizations of the galaxy rebuilt in the following millennia, they were left with only distant historical fragments of the age of Forerunners. The San'Shyuum, initially rebellious against the Forerunners, turned to worshipping their long departed oppressors. Without the complete account of what had happened, the San'Shyuum came to believe that the Halos were great machines designed to ascend its users to godhood. Their interpretation of the fragmentary texts was that the rings summoned sublime energies that burnt away falsehood and freed the soul from the material world. The Forerunners, according to them, had become spirit and reined as gods, and had left their relics and holy texts behind to inform the other races on how to ascend like them. This legend about the Halos came to form the basis of the Covenant religion, and the Covenant was formed as an interspecies union that together would seek the sacred rings.

There were some doubts among the Covenant that the legend was true. Some Sangheili, such as the clan of Ussa 'Xellus, believed that the rings did not exist but were a lie made up by the San'Shyuum in order to deceive species they enslaved. One of San'Shyuum Prophets, Mken 'Scre'ah'ben, had doubts about the rings' function, since to him the descriptions of the Halo effect sounded close to that of weapons. Another San'Shyuum, the future Prophet of Truth believed in the rings and initiated their firing, but feared that some of the faithful could be left behind and not ascend, after discovering that humans were, by his interpretation, un-ascended Forerunners. Truth thus resolved to eliminate all opportunity of doubt for the Covenant, demanding the extinction of humans before their status was discovered then ordering the Sangheili assassinated and replaced with Jiralhanae when many Elites began to question the claims of the Prophets.