The Yoort resurgence
The Yoort are a race with simultaneously ancient and recent roots. Supposedly this race is a force which once spread and held dominion across much of the galaxy, having successfully conquered it in the ages before the rise of mankind and become one of a long list of galactic powers that the Eldar viewed as beneath them, for all that those powers occupied a much larger area. An impressive accomplishment for a race of small, almost insensate slugs. When the Yoort were at their peak they came into conflict with the Eldar Empire, having come to believe themselves great enough to challenge the creations of the Old Ones, and they were cast down for it.
Their industry and technology, stolen from a thousand other races as much as invented themselves, was proven insufficient, crushed and broken in the face of the Psykic Might of the Eldar Empire, their worlds and secret places burned, cursed, fallen into the warp, or leveled. Then the Eldar turned their precognitive power to preventing them from ever rising again, seeking out and destroying their hidden populations, caches of gene-stock, and databases of technology. This was a difficult task, for the Yoort are very nearly soulless, and halting any hint of their method of reproduction requires draining the pools in which they breed and filtering both the water and silt for any sample of their algae-eggs, and worse, requires checking for hidden machines meant to recreate the algae-eggs after the total annihilation of a planetary population. This was a common tactic of the Yoort, allowing them to wait until enemies were certain that they had cleared out a planet's biosphere, before automated systems began training infiltrators behind enemy lines. As such, these automatic rebirthers were present on many worlds, often accompanied by technological databases. When taken alongside cases where the Eldar simply missed a few of the Xenos, the distraction of the Eldar from the purge once it was clear the Yoort were beaten, and the sheer scale of their empire, with over a million worlds to their name across the galaxy, it became inevitable that some of them would be missed by the search-and destroy efforts. Though those populations who were missed were often left helpless and without the means to rebuild for centuries, millennia or even longer.
A skeptic might be wondering how a race of slow-moving, blind, nearly insensate slugs who seriously struggle with complex language in their native forms could amount to much and dismiss them as a threat. Such a skeptic would almost immediately be taken by surprise and possessed by the Yoort, their brain taken over to provide a meat-puppet and a host for the Xenos, leaving the fool to helplessly watch as another creature moves their body, rummages through their private memories, and lives their life. Despite this pervasive access to their victims minds the Yoort are not psykic, and can only rarely keep control of psykic host creatures if they are unwelcome. Instead they utilize a varied suite of physical abilities to penetrate the braincase and to manipulate or read the motor, sensory, and memory cortexes of their target's thinking organs. Their distant ancestors apparently went through repetitive cycles of targeted genetic engineering to give them the array of abilities needed to successfully body snatch creatures that did not originate on their home world, to the point where it can be assumed that any modern starfaring species is a potential host target. Only those Yoort populations that have degenerated through mutation are less indiscriminate in their potential threat. There are even rumours some combination of genetic alteration and use of lost technology had given them the means to take the Eldar as hosts, and that it is this development that provoked the war of their destruction.
The Yoort never really recovered from the lack of possible hosts alongside many of their surviving populations, the sudden Eldar strikes against any group that seemed to be recovering, and their own new-found disunity, they slipped into isolationism and obscurity, dwindling with each passing century, even with more of their worlds being inhabited by potential host species.
It was, ironically, the rise of the Imperium that reversed this Xenos species weakening. While Eldar interest in hunting prey that lacked the means to thrill or challenge them had been dwindling for millennia, to the point that the sudden destruction of the Eldar Empire did not significantly improve the Yoort situation, the opening of galactic trade lanes under the Imperium greatly aided their situation. In an amazingly risky infiltration, made easier by occurring before the Inquisition could rise to power or proliferate across the Imperium, they turned an unknown amount of the Imperium's Chartist shipping and Astropathic messaging organizations to their cause, regained contact between their widely-separated surviving enclaves, then set about obsessively covering their tracks and increasing the number of intermediaries and the legitimacy of the intermediaries that stood between them and their manipulation of Imperial organizations.
The Yoort gained knowledge of numerous other Xenos species through the cat's paw of the Imperium's Rogue traders and began taking them as hosts, avoiding allowing themselves to give themselves away to any force that could could coordinate their galactic annihilation, having learned their lesson about avoiding direct confrontation with galactic powers. Over the Era of the Imperium they grew from an initial surviving thirteen enclaves to a scattered empire of numerous worlds.
Unfortunately their dependence on the Imperium to connect their widely separated holdings left them vulnerable to the it's collapse, as the Imperium became increasingly dysfunctional and fanatical near the end of its existence they found their ancient system of exploitation more frequently challenged by those requiring the agents it depended upon to demonstrate their devotion to the Emperor and take on tasks that distracted and delayed them. By this point there were so many layers of infiltration between the Yoort and their supply chain agents that they could not track the activities of their own logistics and they had long lost the expertise that allowed them to create the system in the first place to old age and death. They attempted to crudely intervene to rebuild their logistics, and in doing so alerted the Inquisition, the Yoort leadership freaked out, but also smelt blood in the water with the reluctance of the Inquisition to openly confront them, taking it as a sign of the weakened state of the Imperium, its frantic purges of external forces, and paralysing witch hunts for internal threats that, as often as not, didn't exist. They managed to rally the Yoort behind them into what they promised would be a steady infiltration, followed by a quick, victorious war regaining their lost logistics through captured human personnel, promising to fulfill the traditional unifying goal of the Yoort and devote themselves to ensuring that every Yoort in their teeming multitudes could take a host. The effort wasn't quick, or victorious.
The war devolved into increasingly stubborn fighting on the human's part, and the infiltration into frantic suspicion of their neighbors, and even more severe restrictions on the activities of Imperial citizens, until any deviation from the restrictive holding pattern of society was met with death. Numerous Yoort planets were subject to Exterminatus, exposed to a lethal drying-disease, with even those submerged in water shrivelling, cracking, and turning to dust, invaded by fanatical humans, supplied with addictive drugs or chemical attacks that rendered Yoort insane and unable to leave their hosts, or eventually, fallen to internal schism.
With the birth of the Abomination the most traditional elements of Yoort society fell into corruption and were purged in a civil war, tainting their views in the public consciousness, and the snuffing of the Astronomicon and death of the Astropaths gave lie to their goals.
The already physically separated domains of the Yoort became politically separate as well, reacting against the Abomination, or against humanity, or against the 'cowardice' of their former leadership in not striking the Eldar in their millenia of weakness, or against the cruelty of their previous host taking practises. Seeking to fulfil their need for a unifying enemy, or vengeance, with a war they can all agree on, or for purity of their souls, with a struggle against themselves that they realize they need, or to allow their sympathies for those they spend their entire lives with run rampant, with calls for a more voluntary relationship with their long-term hosts.
Often more than one of these things at once.
Technologically, they mostly use tech copied from various races over the age of the Imperium, but their equivalent of explorator groups have retrieved or recreated a few pieces of technology from their ancient height and the ancient infiltration systems that are the root of their survival: stealth systems, hidden manufacturing systems, the smallest craft mounting a warp drive in the entire galaxy, barely larger than a fighter, and expensive personal energy weapons with absurd armor penetration called Drake beams are all throwbacks to these ancient days.
Spiritually, they believed in the cultivation of an immense ego, unable to stand being compelled or commanded, as a defence against the powers of the Warp. Those factions that seek a more voluntary relationship with their hosts compromise this, and alternatives relying on uniting the wills of two beings at once are not yet mature in their development.
@Durin, a thing, yes this is a blatant rip off of the Yeerks from Animorphs, but the setting requires more horrible stuff in it, and adding an invasion of the body snatchers was completely necessary.