Given that you will almost certainly need to scale up the power level of combatants to account for Akivasha, I would like to put in a suggestion for villains that Cain could encounter in the future:
The Sons of Darkness
Chaos cares little for morality, having no more regard for the supposed intrinsic value of human life than the Imperium it opposes. But even so, there are limitations to what even the Slaves of Darkness will tolerate amongst their ranks. Unspoken laws that even their incomprehensible masters allow and encourage. Lines not even the Prince of Excess will tolerate the crossing of. These uncodified doctrines bar precious little, and it takes feats of depravity and sadism that outstrip even the madness of the Warp in their inconcievable scale to break them... but they exist, and they can be broken. The Sons of Darkness, a Warband reviled even by the forces of Chaos, stand as living proof of this unlikely truth.
Introduction
Very little is known of this loathed Warband's origins and past deeds. No records exist of a Chapter, Loyalist or Renegade that bore their name or heraldry at any point, and no Chaos Warbands would acknowledge that this band of rogue Astartes had been birthed amongst their ranks even if it were true. This seems to hold true even amongst the ranks of the Sons of Darkness themselves, for they care not about glory, or honor, or the past, and speak little on the subject of all three. The same can be said of the Warband's organization and homeworld. The Sons of Darkness sometimes reference a "Master of Darkness", but they have never elaborated on the context behind that term, at least not to outsiders; whether he is a deity they worship, a flesh-and-blood being serving as the Warband's leader, the protagonist of some fabricated tale they spread amongst themselves, or something worse than any of the above remains unclear. What is well-known, however, is their goals, and their beliefs; these things, they are all too happy to share with others, through both word and deed.
At a glance, their belief system reads like some absurd inversion of the Tau's concept of the Greater Good, equal parts ludicrous and abhorrent in goal and scope. If they are to be believed, all acts that they define as 'good', all expressions of empathy, heroism, and selfishness, are all violations of the "natural order" of the galaxy. In their twisted worldview, to be anything short of a knowing and willing epitome of evil incarnate is an act of blasphemy against the very fabric of reality. Thus, they seek to correct these "blasphemies" through sheer, overwhemling 'evil', to inflict as much suffering and misery and pain as possible on as many sapient and intelligent beings as they can, drowning out all 'goodness' in the galaxy through the overwhelming weight of their vile, monstrous will.
Knowing this, it's little wonder why they are so deeply hated even by the likes of the forces of Chaos, why they have managed to achieve the impressive feat of being rejected by the likes of monsters like the Night Lords and Emperor's Children on account of being too monstrous, even for the likes of them. The forces of Chaos, with a few notable exceptions, frequently stoop to similar depths of cruelty and sadism, but these vile acts are always a means to an end. The end may not be noble, and the means nonsensical and of unclear contribution to said end, but even so, such acts are never a means unto themselves... unlike with the Sons of Darkness. For them, their means and the end they seek are one and the same, their vile acts breaking the minds and souls of those subjected to their tender mercies, driving these broken souls to commit further depravities and break others beneath the weight of their evil; a vicious cycle that they will continue to propagate until the whole galaxy is just as pointlessly sadistic and needlessly cruel as they are.
Behavior
The Sons of Darkness are frequently encountered throughout the Damocles Gulf, and seem to take particular offense at the presence of the Tau and their utilitarian, egalitarian beliefs. Beyond that, precious little is known about how the Sons of Darkness wage war, and what is known only serves to make them even more baffling. The number of Astartes who will engage in a battle varies wildly, from as little as a single Obsidian Soul to several hundred Astartes. Regardless of how many Astartes appear, it often seems that absolutely no attention is paid to supplies, reinforcements, or other operational concerns; infiltration is attempted if the opportunity presents itself, but otherwise, an Obsidian Soul will simply strive to spread as much misery and despair and pain as they can, doing so in whatever way their madness-driven whims dictate. Again, it seems they pay absolutely no attention to the condition of their gear, the strength of their foes, or even basic self-preservation. Were this any normal Warband of Chaos Space Marines (insofar as the term "normal" is even applicable to the servants of Chaos), such behavior would have seen the lot of them wiped out in a matter of weeks.
And yet, somehow... they are not only surviving, millennia after they were first encountered; they are thriving. The atrocities they commit ever grow in scale and frequency, their numbers and resources swelling with no discernable cause in flagrant violation of all sanity and reason. Nobody knows how this is possible for a Warband that rejects all allies in this way... but there are theories. Theories backed up by what those who oppose them have witnessed.
Curses and Misfortunes
Stories abound about these obscure and vile Astartes, stories of seemingly mortal blows rendered pointless as the injuries they inflicted vanish into thin air, of profane strength conjured from no discernable source, of impossible feats achieved by the Sons of Darkness, reminiscent of the Sisters of Battle's Miracles yet amplified to unbelievable extremes and turned towards a cause so vile it leaves even the Gods of Chaos disgusted with them. Some choose to believe these stories pure fabrications, or at the very least heavily exaggerated... but nobody can deny that there is something different about these particularly deranged Astartes, something beyond their horrid beliefs.
All who face the Sons of Darkness in battle find that things inexplicably go their way, as if fate itself were buckling beneath the sheer weight and magnitude of their twisted willpower. Their foes suddenly find that their most rigorously tested and maintained weapons suddenly become inexplicably likely to jam, break, or even outright backfire with lethal consequences for the user. Master-class sharpshooters who are in opposition to their cruelty suddenly find that 95 out of 100 shots they fire miss so wildly they sometimes end up hitting their allies instead, and that the 5 shots that don't miss rarely inflict anything more than scratch damage. Tempers flare and reason crumbles among the leadership of the Sons of Darkness' opponents, to the point that even the keenest commanders start making horrifically bad judgement calls that they later realize are nonsensical, counterproductive, and sometimes even outright suicidal.
Meanwhile, the Sons of Darkness experience the opposite. The chances of their weapons breaking or malfunctioning are so minimal they are nigh-nonexistent, and in the rare event it does happens replacement weapons all but fall into their laps. Shots they fire almost always hit something, and while it is not always what they were aiming at it is always something that inflicts a measure of pain or suffuring on at least one of their opponents. And attempts to predict their movements almost always end in disaster, their madness somehow prompting them to unknowingly do exactly what they need to do to throw a spanner (or twenty hundred) into the plans of their foes. Attempts to see the future through arcane means are outright guaranteed to produce disastrous failure at absolute best; even the vision of Tzeentch himself often fails to correctly discern what these madmen will do next, a fact that enrages the Changer of Ways to no end.
Reality doesn't outright break beneath the weight of whatever curse or blessing causes this effect; all these incidents are always possible, albeit incredibly unlikely. In isolation they can be easily be explained away as basic misfortune or unlucky coincidences. And the Warp is nothing if not fickle; even Tzeentch has been known to be wrong before. As is his nature, some of his plans do conflict with each other and are thus doomed to fail, and while the possibility that this is somehow true for ALL such plans that have anything to do with the Sons of Darkness is so slim as to be effectively nonexistent, it is still a possibility. But taken altogether, in light of the way Chaos, Imperium, and Xenos alike find that any battle fought against the Sons of Darkness (or even fought in their general presence) collapses into an ever-escalating series of FUBAR clusterfraks to end all FUBAR clusterfraks... the final results are so blatant, so extreme in their sheer, undeniable scale, that not even the most thick-headed beings can deny that the balance of destiny is somehow rigged in the Sons of Darkness' favor, the dice weighted, loaded, and shaved to the point that they are all but guaranteed to yield results these deranged madmen approve of. How, or why, nobody - not even the Changer of Ways himself - knows.
AN: Before I begin discussing these Complete Monsters at length, I'd like to talk a little about "Grimderp". It's a term frequently thrown around with regards to 40k, but it's applicable to plenty of other media, particularly World Gone Mad settings like Invader Zim or Judge Dredd. Basically, it's Grimdarkness that's Played for Laughs, intentionally or otherwise; circumstances so overwhelmingly dark they end up coming off as depressingly ridiculous rather than depressingly serious. Which isn't necessarily bad; this Grimderpness contributes a fair bit to what makes WH40k attractive. But in excess... it can become a problem.
So what does this has to do with the Sons of Darkness? Simple; they are a commentary on the unnecessary Grimderp 40k sometimes succumbs to. They are a group of people who have looked long and deep into the grimmest, darkest depths of the 40k universe, looked at the Night Lords and Emperor's Children and Iron Warriors and all the other countless forces in the universe whose acts of evil are beyond the pale even by 40k's absurdly high standards, looked at all the evil and despair and cruelty and sadism rampant in the setting... and promptly declared "Nope, not grimdark enough. Not grimdark enough by HALF." They are the "grimdark singularity" that Zahariel described Erebus as in this lovely little sidestory, stripped of all pretenses of logic, reason, or higher goals in favor of a bold, blunt admission that they exist solely to make the setting they exist in as unrelentingly dark, pointlessly sadistic, and utterly devoid of redeeming qualities as they possibly can, an admission made both in-universe and out. They are the living embodiment of the abandonment of internal consistency in favor of further darkness, walking, talking, torturing manifestations of Grimderp ridiculousness taken to such horrendous extremes their antics end up crossing the line three times, circling from horrifying to silly and then all the way back to horrifying in their sheer scale.
And as for the name, and the "Master of Darkness"... that is a subtle nod to the main villain of the Samurai Jack cartoon, a lovely fellow by the name of "Aku". An expy of him would, in some ways, be the perfect patron for a group like this, as Aku in Samurai Jack is a being so utterly steeped in his own evil he often finds himself choking on his own sadistic streak, making stupid and unreasonable decisions that ultimately result in him screwing himself over because he simply can't resist the opportunity to gloat, to torment, to do everything in his power to amplify the suffering of everyone around him. He's Evil, pure and simple, unrestrained by any rationality, self-preservation, or even basic sanity. I'd even considered giving these guys black, red, and green color schemes and making their symbol a hexagon with six horns (in reference to Aku's head and it's shape), but left those parts out in case Zahariel wants to use these guys and has a better idea for how they might look.
And on that note, Omake authors are free to use these nutjobs, or expies thereof, in their stories... and that goes double for Zahariel. This fic is already shaping up into a Vampire: The Masquerade fusion crossover; why not import a living mass of evil incarnate to add to the mix?