There's one place you can always go for all sorts of Ancient Evil... the Orgoth!
It wouldn't be a stretch to say that some dark cult or somesuch unearthed and deciphered some books/tapestries/etc from an old Orgoth site of imprisonment/worship/horrible experimentation/etc and are looking to complete the rituals to bring about the terrible evil prophesied.
The Terrible Evil itself could be a trio of Orgoth powers, or (perhaps inaccurately) the cult may interpret the 3 to be Scions of Thamar or Aspects of the Devourer that they are trying to unite into a superdeity.
You could even throw the Blackclad in there (especially if you use a Devourer hook, as a number of druids revere the raw natural power of the ole worm). Perhaps the ancient wars fought by the Wind Dukes were battles between druids of old and the Orgoth ... or Infernal servants of the Orgoth ... or some other menace even predating the Orgoth. The tomb could be that of a druid who attained great power.
The Kyuss role... hmmm... could be a Scion looking to trick some mortals into performing a rite to grant him/her some kind of power. Heck, could even be a regular living mortal who has found some way to orchestrate all this. Perhaps an Infernalist who was promised some great knowledge/power by the Nonokrion if he can get a bunch of folks to do the ritual. If the Kyuss character came long after the Orgoth, he may have arranged to plant the information 'discovered' by the cult. Heck, this whole thing could have been concocted by some Infernal(s) and played out over centuries by manipulating various mortals who sought power through them. If you really want to get wacky, Toruk could even be behind it all somehow.
Sure, IK may seem 'narrow', but there's plenty to work with when you're trying to convert adventures like this as long as you aren't too entrenched in the idea that 'this char in original adventure is an Outsider, so IK char must also be outsider' or the like. I know this came out very rambly, but I'm just trying to throw out ideas. Using various combinations of just the stuff I mentioned, I could probably come up with 3 or 4 completely different scenarios for the Age of Worms to be happening on Caen.
Just because there isn't a Demon Prince of Unpaid Library Fines or God of Some Specific Subschool of Magic doesn't mean you can't find motivations for the powers/nations/organizations/individuals of Immoren to pursue similar goals to the Infinite Outsiders of traditional D&D.
Wow, I was just proofreading this and it hit me -- the goal of the cult is to unite 3 of Toruk's spawn into a superdragon! The whole heartstone concept meshes well with this. If you use dragons, though, remember they're not as widespread or well known as they are on most D&D worlds. Their powers are largely unknown by humanity and there aren't organizations or prestige classes built up around them because they so rarely interact with humans. They are, though, a pretty good stand-in for the role of a 'lesser god', perhaps even worshipped as such by some remote tribes of non-humans.
Elves! A splinter group of evil Iosans could be attempting a ritual to bring back 3 of their gods and merge them since alone they would not survive returning to this world.
I'm sure I could ramble some more, but I need to get to sleep sometime ;)
Good luck!