On their own, vemeraks are powerful subterranean horrors. But they are also harbingers and hangers-on—the pilot fish and remoras of truly titanic monstrosities. In Golarion, such beasts are the spawn of Rovagug; in your world they might be behemoths, Colossal (and likely mythic) chaotic evil dragons, kaiju, or the greatest aberrations. (Atropals from 3.0/3.5 would be another great set of candidates for vemeraks to be found in the vicinity of.) So if you find a vemerak underground in some impossible hidden realm, kill it. If you find a vemerak aboveground, kill it quickly so you can run from whatever worse thing is on its way.
Also, the vemerak is truly an aberration. An acid breath weapon that devours the living but leaves undead untouched? A spell-resistant spore cloud? The ability to turtle its limbs beneath it and create earthquakes? These are not natural creatures in the sense any druid or sage would recognize. This lets you, the GM, put them in weird places and death traps with little to no regard for biology—pure vileness might sustain them as well as any mortal food.
A fiendish thunder behemoth walks the Rivermarch, calving vemeraks from some dark recess as it goes. A troop of angels arrives to head off the behemoth, but their divine mandate will not allow them to contend with the vemeraks, because of the mortal cultists of ruin who quickly flock to them.
The Company of the Marlin believed they’d found a way around the vemerak of Jaxon’s Sepulcher: die. The explorers had a minor artifact that would turn them into wights and back in relative safety, allowing them to bypass the vemerak’s acid breath. But they neglected to factor in the reason behind the vemerak’s presence in the first place: a geyser of dark energy that overpowered the feeble artifact and ensnared the temporarily undead company permanently. It warped them as well, so now anyone who gets past the vemerak must also face a three-headed devourer.
The tower elves of Tal Meigh (see the Advanced Race Guide) have studied vemerak sightings for years, and they have extrapolated a pattern not unlike the mystic circles the elves themselves use in their callings. They also whisper the name of a scorpion-like beast of legend said to sleep not far from the heart of that pattern: Bezravnis.
—Into the Darklands 62–63 & Pathfinder Bestiary 2 278
Full details on the vemerak can be found in Into the Darklands, a glorious book from Golarion’s 3.5 days. Bezravnis is one of the new kaiju in the Bestiary 4; we’ll cover it soon enough.
By the way, if you’re looking for the velociraptor, we’ll get to it when we swing back around to the styracosaurus…so you have plenty of time to get used to the fact that it has feathers…