The most surprising thing about The Koronus Bestiary is that one of the creatures isn't trying to kill you. The Mukaali are a bright spot in the WH40K universe, just loyal pack animals with an unusual resistance to desert conditions. But don't worry, it's a one-time lapse. Everything else is definitely trying to kill you.
Which, fair enough. It's a monster book. Even in settings much less hostile than WH40K, those tend to be biased towards the dangerous. What really determines the quality of this kind of book is whether these monsters are diverse and memorable enough to anchor adventures. And for the most part (Mukaali notwithstanding), they are. You've got pack hunters that are born from nightmares and can walk through walls, psychic ghouls who are eternally dying of thirst and subsequently drain the fluids out of their victims, birds with feathers made of monomolecular ceramic blades, and also at least one big, dumb claw beast. Oh, yeah, and a fucking SPACE KRAKEN. I'm not sure if I could have ever forgiven The Koronus Bestiary if they'd forgotten that one.
Where the book loses me is in the long middle chapter devoted to intelligent xenos threats. In a way, this is a reasonable addition. The Monstrous Manual had humanoid enemies, and that's pretty much the model we've all been working off of all these years.
I guess what bothered me was that The Koronus Bestiary largely covers species we've seen before. Nearly all the material on Orks, for example, is redundant with Into the Storm, though we do get a new weapon that is basically a handheld warp portal that allows Orks to teleport snotlings (foot-tall mini-orks) on to their enemies. We also learn a few interesting things about Eldar culture, like the fact that they accidentally created one of the Chaos Gods (though I'm sure I've seen that somewhere else) and that they now adopt a rigid system of professional specialization to ward off her temptations.
But the Rak'Gol and the Stryxis are still terribly mysterious, since Rogue Trader books are deathly averse to presenting anything but the human perspective (and a highly authoritarian and violently xenophobic human perspective at that). Alien motivations are supposed to be "unknowable" and "incomprehensible" And thus, when the books give advice on how to run them as NPCs, it winds up amounting to not much more than "they do pointless-seeming shit for no apparent reason. Deal."
I think they think it's a horror trope. Nothing is scarier than the unknown. That's why we've decided to keep GMs in the dark and show them nothing more than a few characteristic encounters. Too high a degree of abstraction would ruin the mystery and tempt the GM into running them as if they were people.
It's kind of funny to think about them writing the Imperium like that, as the inscrutable aliens whose motives cannot be discerned by reasonable people,
"Every time we encounter these greasy bipeds with the giant shoulders they immediately try and kill us. The keep screaming about how they need to 'cleanse the galaxy of xenos filth.' Why don't they at least try to coexist?"
"Maybe it's their religion."
"That's a little pat, isn't it?"
"You're right. I guess the alien mind is impossible to understand."
What this version of the setting seems to forget is that it's not necessarily the case that in the WH40K universe everything is trying to kill everything else, but rather that all the major factions are one variety of evil or another because in the WH40K universe, if you're not a raging asshole, you're destined to be ground mercilessly under some assholes boot. The boot, in this case, being Rogue Traders.
And while I will acknowledge that a monster book isn't quite the best place for it, I do wish that at least occasionally, when the books casually talk about the Imperium committing genocide, at least some of the victims would be demonstrably innocent.
I think that's why I projected so much on to the Yu'Vath. Their story is that they are an extinct species, wiped out by St Drusus' crusade into the Calixis Sector (the last frontier of civilization before you get to the "unexplored" region where Rogue Trader is set). They were a cruel and decadent species who enslaved other sapients and dabbled in technology that manipulated the warp. Their leftover inventions include devious and deadly security constructs that were among my favorite of the book's entries.
So, based entirely on nothing, I took to assuming that the book's take on them was just imperial propaganda. They didn't genetically engineer the Byavoor to be docile sacrifices for their dark religious ceremonies, that's just the Imperium deliberately misinterpreting the treatment of certain condemned criminals. The species of crack pilots and the near-microscopic crystal people associated with the Yu'Vath voluntarily, as part of a federation of younger species that centered around the mentorship of these clever and curious ancients. The Yu'Vath were guilty of no more than a willingness to experiment in technologies humans are too timid to touch.
Like, maybe the only reason that the Crystalwisps steal human memories is because they're programmed to consolidate all information into their crystalline memory matrix, and that programming has never been altered to recognize humans as any more sapient than the computer archives, books, scrolls, and murals it's also been known to absorb. Maybe the reason so many people are transformed into sandslime is because humans never bothered to heed the giant "warning: industrial-grade nanopaste" warnings wherever it was found.
As I said, this is supported by absolutely none of the text, but it comforts me to think that these genocidal fascists can occasionally get it wrong.
Ukss Contribution: The water stealing ghouls know as the Unquenched are psychics who defied the rule of their local Priest-Kings and were punished by being forced to drink from a mysterious spring that inverted their powers and trapped them in a gradually-decaying cage of flesh, driven by their unceasing thirst to pursue moisture wherever it could be found. I can probably do something with that.