So the Khalkos is a cool monster concept, and does seem clearly to act in a similar design space to the Mind Flayer, a hive mindish alien entity that subverts and takes over a local culture....but in a nicely distinctive way.
It has a lofty resume, basically a world eater, in which one Khalkos is normally deemed sufficient to take over an entire world, and "very few worlds have managed to stop them". Does the creature hold up to that?
So its a CR9, that can create 2-3 more CR9s every 13 days or so (about 10 days to spawn, a few days to grow to adulthood). So yeah left in the right conditions, I could see a swarm of these guys being out there, and a hundreds of CR9 style creatures would be a very scary threat.
But....how would that actual happen? The first issue, the supply of aligned creatures. Most worlds don't have all that many aligned people, even paladins and heralds aren't aligned by default. So while I could see these guys spreading on maybe the planes of hell, I don't see how one of them could ever grow that big on a standard mortal world. Now they can infect non-aligned people to do their bidding, but with a variable time limit before the person dies..... that's a major drawback in your insurgency plans. Bodies create investigations, while mind flayers can keep key personnel constantly dominated to throw off such scents, Khalkos are constantly churning through their insurgents....that is hard to do and keep things secret.
The next big weakness, this is a DISEASE effect. Diseases are easy to take care of for mortal communities let alone celestial ones. Detect Disease is a level1 ritual (though maybe levelup has changed it), and I would expect any religious community worth a Khalkos' time would have that ritual and use it to ensure good help for its members, which would be instantly flagged. Most celestial communities have access to greater restoration. And gods can be infected?.... I can't imagine that any god wouldn't be immune from disease, or at least could easily cure it with a wave of their hand. Ultimately this seems way too weak as a disease to do the kind of damage described in the flavor. If nothing else, it should probably have a clause that normal disease immunity doesn't work against these things (else the gods would just send an army of paladins to take these guys out). Also, if the disease can't be detected by standard means of detecting disease, that would add another great irony.... the communities that rely on detect disease magic would likely not use "medicine" and such....and so their arrogance in their power would play right into the Khalkos' hands.
So funny enough, what the mechanics tell me.... is that these guys are actually the scourge of devils and demons. With no natural resistance, detection, or curing of disease, and with plenty of aligned fodder (aka lemures and the like) to use as breeding ground, I could absolutely see Khalkos as Asmodeus' greatest nightmare. And that would be kind of a fun twist, should we protect the evil demons from getting consumed by the swarm because something worse will emerge? Have a blood war fight, only to see the landscape shift as demons are being replace with Khalkos spawn... that would be cool. But I don't think I could see these kinds of the "destroyer of worlds" flavor like a mind flayer, they are too "picky" for that to work.
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