The Asynkrian Paradoxy
The Asynkrians are perhaps the oddest xenos species that the Imperium has faced, not because of their culture, which is comprehensible, if reprehensible in its taste in music, and its alienness, nor their form, which maintains a relatively pedestrian tripodal symmetry, also reprehensible, but rather for their mindset.
The Asynkrians cannot consciously determine the difference between memories, plans, and present events, relying on reflexes to keep themselves reacting to immediate threats rather than past or potential threats. The Ordo Xenos believed this has something to do with precisely how their minds evolved the ability to plan, achieving the trick in a way that is decidedly non-standard among galactic life as the Imperium knew it.
While for the most part this is not a special concern, it makes them even more incomprehensible than ordinary Xenos races, and has influenced their technology to develop in threatening directions.
Rather than developing a means of sending their ships into the Warp to achieve faster-than-light travel like any other species making their first forays into the warp, the Asynkrians developed a means of sending their ships into the warp to achieve time travel. This trick of techno-heathenry is of great use to defensive or avoidant efforts, explaining the Imperium's prior inability to adequately annihilate them.
This system provided them with extremely capable precognition, though apparently subject to many of the same pitfalls and counters as psykic precognition, it gave them the ability to borrow from their own future productivity in their efforts to concentrate their greatest weapons against any invader of their worlds, or to provide manpower for colonization efforts, It allowed them to borrow from their own future research in a befuddling and paradox inducing display of explosive technological growth, and it allowed them to spread -slowly- though the galaxy by means of moving forward or backward in time until another system intersected a point they could reach in space.
In the handful of encounters the Imperium is aware of having with them they became known for a nonsensical logistical capacity, leaving some systems undersupplied despite being heavily industrialised and surrounded by other systems claimed by this race, and others isolated in the middle of nowhere or imperial territory constantly well reinforced. They would seem to have perfect knowledge of groundside plans as long as the loss adverse admirals of the Imperial Navy allowed them to maintain any fleet presence, would have ships appear from the future and past for a battle, only to blink out of existence(or more bizarrely, not blink out of existence) if someone got a lucky shot on a past version, and could be sore losers by sending hidden colonization fleets back before the Imperium became aware of their presence in a region or forward to long past the point that the Imperium was certain that they were gone, made worse by their mostly-increasing access to stealth technologies. The only saving graces of conflict with them were their dependence on ships, even greater vulnerability to warp travel disruptions than the Imperium, division between many small polities(or paradoxies) and greater ability to send ships forwards than backwards in time.
They continued to be a terrible annoyance for a literally indeterminate amount of time, being encountered with both Chaotic and non-chaotic allegiance, before suddenly being bumped up to 'threat to the Imperium' status with their successful reverse engineering of another race's warp drive, unifying and driving to refugee status any dissidents with imperceptible speed, theoretically being capable of menacing Imperial worlds, then suddenly disappearing.
Apparently this is the result of the potential actions of the nonexistent Ordo Chronos, successfully annihilating much of the species in one timey-wimey swoop and convincing the survivors to seek greener pastures before and after the Age of the Imperium. It is difficult to say what happened to any who went back, beyond almost certainly being destroyed by the Emperor, or potentially inspiring their own earlier development of time travel technology, and any who went forward would have suffered misjumps or failures of their protective fields if they were in the warp at the time of the snuffing of the Astronomicon and birth of the Abomination. Though some could have survived even that disaster and others could have dodged it, after all, their few psykers, otherwise weak by human standards, are potent pre-and-post cognitives, and might have warned them out of the warp.
The only reason the Imperial Trust even knows of them is a commendation in Governor General Aelfric's file from far before his birth for being instrumental to their defeat in an engagement, though he certainly remembers no such battle. There is no way of knowing what that means, perhaps his memories were erased to avoid paradox, as he admits certain members of the Timeless Army have had done, perhaps it did not occur in his personal causality, with the events in question involving a parallel Aelfric or someone else with the same name, it is possible that the entire thing is a trick and was never based in any vaguely real event, or perhaps these events just haven't happened yet.
The Inquisition assumes that they are a real and causality-depending-present threat.
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@Durin more xenos species ideas and a question
Balance? What is that? Can you eat it?
But more seriously, this could be balanced by them using their somewhat broken advantage to grow into a major threat(or at least survive in this era), and making their time tech absurdly difficult for human minds to figure out.
Alternatively, this becomes a plot hook for an Omake that has the Timeless army emerge from warp larger than they entered it, having visited the past, fought the Asynkrians and refilled their ranks with younger soldiers there, before returning to the present.
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