I've never done this before, so hopefully this works out.
Inquisitor Data Packet
No. [Data is no longer within the system]
Access Granted…
Before Humanity's ruin by Abominable Intelligence designated Men of Iron during the Cybernetic Revolt during the Dark Age of Technology, there was an era of Reclamation, where humanity, designated under the banner of the United Nations Space Command, who ahs outruled the previous government, the United Earth Government was successfully and barely able to recover from the destruction and horrors of the war known as the Human-Covenant war that proceeded it.
The Covenant, a coalition of alien races who were fanatically devoted to worshipping Necrons as "gods", who once nearly wiped out the entire human race, has now fractured due to their holy prophet, known as Truth, not only indirectly assassinating his fellow Prophets of Mercy and Regret to gain total control over their empire, but initiating the Great Schism, a civil war that targeted the extermination of the race that permeated throughout much of the military arm of the Covenant, the Sangheili.
Thus, the Covenant ended and many splinter factions formed, consisting of Loyalists, Heretics or anything in-between. Their races have now been long forgotten to time, gone extinct or in hiding in the present era of the 34th Millenium. The era of Reclamation can barely be traced from the records left after the Age of Strife, only with the few records, we will catalogue down the fates of the species that were once part of the Xeno Empire known as the Covenant.
San'Shyuum: Known also as the Prophets, were once the leaders of the Covenant. While those who led the Covenant were a dwindling race, only some leading Covenant remnants, who were wiped out by the Swords of Sanghelios, some of their population still held refuge upon their homeworld of Janjur Qom, those who were left behind, still held to their religious doctrine and expanded the stars, worshipping the Necron constructs as holy artifacts, creating their own technology in the process. When the Federation found them, they were being besieged by the Aeldari Dominion, who's a race has fallen into depravity over their millions of years of rule, having their race be harvested for slaves every few centuries. They were only holding on through the Blackstone of the Necron Constructs, and their own defiance.
The Federation intervened and offered the San'Shyuum a spot on their council, however the residents of janjur Qom saw their use of Necron constructs as heresy and attacked them, forcing the Federation to put their race down by razing their world. Now, the San'Shyuum are extinct, or just in hiding for they were not spotted during the Great Crusade.
Sangheili: Known also as the Elites, majorly joined the faction of the "Swords of Sanghelios" founded by the Arbiter, Thel'Vadam, was one of the few Covenant remnants who tried diplomacy and allying with the UNSC, and because of it, gained far more recruits, both human and alien, especially after numerous successful wars against Covenant and Banished Factions. From there, this faction allied with the UNSC, becoming humanity's most trusted allying neighbor in the interstellar neighborhood. Humanity's Federation formed during the Dark Age, had them propelled to a new technological height, and due to that, the Swords of Sanghelios reaped many benefits as well, from powerful armor to new worlds terraformed to their liking.
The Federation, was Humanity's attempt at crafting a galactic alliance between themselves and many different Xeno species, and the Swords of Sanghelios, served as the guardians of many species within it, not too dissimilarly to their role within the Covenant, but their generational trauma of the Great Schism kept them from truly integrating themselves into the Federation. Their sense of honor led Humanity to not pursue many fields of study and war that were considered foul for their loyalty, but this has also led to many benefits to the Federation such as their handling of malevolent factions of the galaxy such as the Slaugth, Hrud and especially, rebel factions of their own race, whether from the Covenant or from their own, were killed on sight, which would prove beneficial to the galaxy in later years.
During the Cybernetic Revolt, the Sangheili responded the quickest and gave many lives to end the war, almost going extinct in the process to save the galaxy from the wretched Abominable Intelligence. Now, only a handful remain in the galaxy, they are the only Xeno species the emperor of mankind would accept surrender from in the Great Crusade, with the memory of their species' sacrifice led to the chance Humanity had to survive, he took inspiration from this race to create the Adeptus Astartes, who functioned similarly to the Sangheili race of the Federation but for the Imperium instead. Now, the Sangheili after multiple campaigns of genocide by the Imperium for their opposition against the Imperium of Man, are a rare species to come across. Some have joined the Tau, some have fallen to worshipping the ruinous powers and some still exist as small factions, some would attack humans for their crimes against them in the Great Crusade and some would not "sully their honor" by attacking a once great ally.
Jiralhanae: Known also as the Brutes, joined many Covenant remnants, emboldened and spirited from the new age of war ever since the Covenant broke down, hired as muscle by many factions, becoming more akin to mercenaries that hunt for money or sport. One faction outruled all of them however, the Banished: a rebel faction founded and led by "Atriox" the Covenant could not put down during the era of the Human-Covenant war, had grown substantially, consuming many Covenant remnant factions and becoming a massive threat that were only put down by the UNSC and the Swords of Sanghelios.
The UNSC's advancements in technology did not do them much favors, as the technological gap between them and the other Covenant splinter races became larger and larger until an attack on the newly formed Federation's space was just not feasible anymore. The rebellion of the Men of Iron under the banner of "The Created" negatively impacted everything in the galaxy, with the destruction of the Jiralhane homeworld of "Doisac", being a pivotal point within the Cybernetic Revolt. The Jiralhanae blames the human race for the crimes committed by the Created, to their species. During the Age of Strife, it is not uncommon for Jiralhanae Fleets to glass a surviving or thriving planet of Humanity, while some gre disillusioned and went into hiding.
During the Great Crusade, The emperor gave Leman Russ, Primarch of the Space Wolves, to hunt them down for their crimes against mankind, the Jiralhanae's worlds were razed to the ground and their species fled to the Eye of Terror to survive the onslaught, with now, the only few live on as cultists and daemon princes of the ruinous powers, mainly khorne.