My group is usually only worried about treasure and power, so I have been trying to play up that angle a little bit. I see little information to expand on from CW that would get my players riled up about getting to Benthosruthsa.
In several places the module mentions people that used to worship Benthos as a god. Use that, just be sure to have these dragon-cult types mention Benthos's name all the time, or say that he guides them to do this.
The best way to do this is to screw with their expectations and make sure they know that Benthos (or people acting on the dragon's behalf) are to blame. Do they think they cleared an area and it's "safe?" Not anymore. Do they think they can sell their goods wherever they like? Nope, the merchants have been pressured to not deal with them. Why is the city guard taxing them to enter the city? Cult of Benthos pressured the guard.
If you had to break it down into a three-phase kind of thing, it would go like this:Phase I:
Drop in some Benthos-cultists somewhere (maybe somewhere the party "cleared" and thought was safe) for a normal fight. This is the first taste and lets the heroes know the cult exists.Phase II:
The party gets either gets robbed of valuables, or finds out that some valuables are actually just junk, or finds a cursed item among their treasure with a note like, "This is what you get for taking my stuff - B." This is where you screw with their expectations.Phase III:
The cult offers to meet up in a neutral location (via a sending, or a dead-drop, or something) to negotiate the return of an item or to cut some deal. Go figure, it's a trap. A really, really, REALLY dangerous trap.
None of this is written in the module because I just made it up. You know your group best, and you know what would spur them on. If that sounds good, do it. If not, maybe this gets you thinking in that kind of direction.
To pull this off, you take pieces of the adventure as written and pluck Monster A from here or Monster B from there. Remember that the red dragon-trog on level 5 (maybe?) is descended from Benthos, and I suppose Cinder (the red dragon from the same level) could be as well. The undead on level 15 serve Benthos even after death, and the Thane thinks he's still alive, as does Galdiera Goldmoon and just about all the other NPCs.
As I've said, use locations that already exist in the module. I'd also keep a loose track of what bad guys (especially named bad guys) that either avoid or get away from the party.
Anyway, kinda rambling, but I hope this helps.
And if your party calls b/s on you for doing any of them, feel free to blame me.