Khrave
The Khrave are mentioned fairly often as far as lesser xenos species go, with encounters ranging from the Great Crusade all the way up to the 41st Millennium.
Description
Described as "chiropteran mind eaters". They differ in size to an extreme degree, with smaller members of the species being roughly man sized, and larger ones being over three meters tall. They are long limbed and sheathed in a reflective carapace while their faces resemble featureless black eggs, with no organs for either respiration, ingestion or sense.
They are a mystery to the Magos Biologos, but psychic scholars know that although they have physical bodies, they reside at least partly in the warp and feed psychically.
Each member of the species was powerfully psychic and capable of manifesting warp glamours, shaping weapons out of energy, possession of weak minds, and raising practically impenetrable force fields that reflect impacts at near-light speed.
History
The Khrave began with an entity called the Autochthonar, that was created as a living, psychic weapon by a technically unnamed species prior to the rise of the Eldar, but which is almost certainly the Old Ones during their war against the Necrons and C'tan. Incidentally, the description of its physical appearance given in the primarch book Lion El'Jonson: Lord of the First makes it sound awfully visually similar to the Nihilanth from the original Half-Life. Maybe the author David Guymer, had just finished playing Black Mesa.
Over time the Autochthonar gained sentience and acquired the ability to reproduce, creating a new species, and naming itself the Khrave. The Autochthonar connected its offspring in a gestalt psychic network that grew larger each time a Khrave was born and stood as master of them all.
The Khrave network could also acquire minds of other species that they planned to use to use as a source of sustenance and slaves. The Khrave themselves could possess their victims much like a Daemon, giving them a direct host body to manifest in reality. But rather than empowering them, and twisting them with mutation, the Khrave would hollow out their minds, essentially rendering them living vegetables. Also unlike daemons, while the Khrave were possessing a host in this manner they were also vulnerable to injury and death, and the resulting damage to the psychic network had the potential to send shockwaves that could stun or injure other Khrave.
The Imperium had several run-ins with the Khrave over the Great Crusade, notably the disastrous Seryn Reach Campaign and the loss of 4,000 Ultramarines, and the Compliance of Indra-Sul where Corax managed to reclaim a human world that had been gorged on over centuries, although the human survivors were too far gone to be integrated into the Imperium and were declared Servitide Imperpetuis and all converted into Servitors for their own good.
Later the Dark Angels came into contact with them during the Muspel Campaign. Lion El'Jonson, knowing whom he was facing in advance, as although the Blood Angels had already rendered the world compliant, it was still a feeding world for the hidden Khrave who were messing with the Imperium by possessing weak minds and inciting petty acts of rebellion and vandalism.
The Lion kept the information from his subordinates to prevent the knowledge from being leaked out psychically and set about baiting his trap. He landed on the world and fortified it, bringing more and more delicious human minds into the system, and essentially daring the Khrave to reveal themselves. This would result in a massive land and space battle as the human population and crews started getting possessed and turning on one another. The combat escalated as more Khrave arrived physically with their own fleet, and The Dark Angels responded by opening their Archeotech toy boxes filled with weapons not available to the other legions; by battle's end, they had deployed motherfucking MEN OF IRON (albeit leashed, semi-lobotomized ones) to support The Lion's boarding action which terrified the Autochthonar, and the sheer level of destruction they unleashed on The Khrave was surpassed only by The Lion himself. The Lion was able to draw the Autochthonar into direct combat and managed to slay it after a decisive blow to the psychic network was dealt on the planet below with the destruction of one of their titans.
Although the Autochthonar was destroyed the Khrave as a species still existed and remained at large up until the Imperium of today, though mostly relegated to space beyond the Ghoul Stars. They have had contact with the Star Phantoms (somewhat fitting if the Chapter truly is descended from the First Legion) and with the Storm Wardens.
It is unknown if they still have the ability to reproduce or manage their psychic network without their progenitor, though this would explain why they have not been able to reach their once eminent status ever again. If the Slaugth are the same species that participated in the Rangdan Xenocides, then the Khrave can join them as a wounded enemy with fantastic powers that are only looking for their opportunity to return and feast on the Imperium.