Still more suggestions for specific adventures:
Zenith Trajectory
I think the most important part of this adventure is actually how you end it. Your players may become (or even start out as) suspicious of their employers. After a week (in game) after they return Zenith, have the doppleganger appear to the players as Zenith, but returned to sanity. I had him deliver a speech like:
"Thank you for you efforts in freeing me. I owe you a debt far beyond what my father paid you. The problems between my father and I run deep... I am afraid that we have not yet worked out our differences. We are returning to our ancestrial home in hopes of resolving our problems, and bringing him back to health."
It gives the players a (false) sense of completion, and that they've done a good act. It might even encourage them to keep up ties with other NPCs if they feel "rewarded" with a satisfying ending.
Crazy Jared is a potential wealth of information. If he casts legend lore on Zenith (after they free him), you can give little hints as to what's in store. "Ah Zenith, my long faithful subject, I marked him at birth to be a great influence in Cauldron..." Of course, they will also believe Jared to be quite loony, and whatever he says by this time may just be tossed off as nonsense.
In the battle with Zenith, Zenith kept throwing out some good Star Wars-esque lines to the Paladin: "Yes! Strike me down and take my place of power!" "Bring your hate to the surface Sir Paladin, and show me your wrath!"
It actually unnerved the party.
The Demonskar Legacy
Most important (I think): make sure the players have met all of the key people in this adventure several adventures ago. This is the adventure that sets the PCs on their way to the finish line. Once they meet up with Alek Tercival at the end, there is no turning back.
At the end of the adventure, drive home hard that they can not return to Cauldron. They might want to go somewhere to purchase equipment, and you can let them find a nearby convient city to meet thier needs before continuing.
And last (for now):
Test of the Smoking Eye
Make the most of describing Occipitus. Your players should feel very uneasy about all of it. Ground soft as skin that "bleeds".... forests that look like exposed rib cages or pubic hair....
At the last trial, the characters might ask you what would happen if one of them simply sacrifices himself. A very good answer to keep them off balance, is that the mummy proctoring the test says "Then one of your allies will be chosen by the will of Occipitus." In both cases, the party members deliberated quite sometime before someone decided to throw himself in.
The other thing...
Kaurophon is way too obvious of a potential threat. My first party saw through him so quickly that the end encounter was over faster than Kaurophon could say "oops."
So, for my second party, I simply made Kaurophon a NG sorceror that truly wished to help the party, in hopes of being able to simply live out his life on Occipitus after one of them claims ownership. His half-fiend blood will always make him detect as evil, so the players will still not trust him. (My paladin was so against travelling with an evil NPC, I actually had to have an avitar appear to him and tell him to go and protect Kaurophon.)
Instead, I teamed up a rakshasa with some teleporting demons. The rakshasa learned all he could from the party secretly, then made himself appear like one of the party members (it was an NPC that I had been running along with them, but it could be anyone without any special ability). The rakshas turned invisible, cast darkness on a pebble, then teleported in with a demon to surprise the party. The demon telported out with the character in question, and the Rakshasa simply goes along with the party from that point.
I did the switch after the first trial, but before they got to the fields to start the second trial. If you pull the switch on one of your players, just let him/her act normal with all of their normal attacks, then at the end, take over their character and reveal as the rakshasa.
You don't want your player to cry foul, so have the "real" character travel back to the Cathedral of Feathers, or convieniently find his way to the party. Give that player all the xp he earned while (unknowingly) being the rakshasa to make sure he stays leveled up.
Also, to make the whole thing work well, do things to make sure party members do not trust Kaurophon, but don't let him break the alignment that you give him. He's still anxious to complete the tests. He still listens to every secret as closely as he can. He still tries to urge the players on to greater danger... not to kill them, but becasue he's truly afraid that someone else might finish first.
If you try to switch out one of the players, find a way that that player loses all (or most) of his equipment... especially any weapons that the rakshasa can't use (holy weapons, bows of strength, etc.). Have the switch happen while they are sleeping. That way, the character will have to use someone else's weapons or something, and it won't be obvious to anyone that he's not himself.
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This is as far as I've played with my groups. As I finish the other adventures, I'll post more. But, from here on out, I think the most important thing is that the characters feel that they are losing important parts of their lives as Cauldron sinks deeper and deeper into trouble. And that they can feel the joy of finally defeating the evil that has been using them for so long.