Tomb of Annihilation is a pretty good adventure. There are a good deal of weird WotC-isms in it (have you ever wondered about your NPC guide's exact height and weight? Tomb of Annihilation has you covered.) but on the whole it's a solid adventure with solid wilderness support and solid dungeons. You could run it without a whole lot of elbow grease.
But that's no fun.
Acererak is trapping the souls of the dead in a giant machine and feeding them to an Atropal, which is currently located at the bottom of an ancient dungeon in an abandoned city in the middle of an uncharted rainforest in a distant land. The lead-in is that resurrection spells have stopped working, those who have been resurrected previously are beginning to decay, and you've been sent to investigate it.
Elbow Grease Note 1: Twice as many immortal oligarchs as you thought you wanted.
Tomb of Annihilation comes with the setting presumption that resurrection is common enough in-setting that people are noticing something going wrong. At first, I wasn't fond of this conceit and was considering dropping it entirely (an easy enough task), as I'm not super fond of the idea of resurrection as a commodity, but then I reconsidered.
Resurrection is expensive. It requires a 13th level wizard to cast, and if we're using +Skerples medieval price list, which is nice and abstracted, the component cost is the equivalent of 100,000 US dollars.
(For extra math fun, calculating the value of 1gp with the current per-ounce market value of gold, times 16 ounces per pound and divided by 50 coins per pound, we end up at $205,760 for a resurrection spell if a GP is only 50% gold.)
This is on top of the fees for hiring a 13th level necromancer, which could easily be ten times or more the cost of the components.
In short, it's bloody expensive. The only people who would even have a chance to access to it would be the rich and powerful, who would be able to become more rich and powerful by installing a revolving door to the afterlife.
The people who are sending you on this mission will reap all the benefit, despite doing none of the work (the book itself says that they've already sent people to Chult and failed. The only actual payment you receive from your patrons are 50gp up front, and a single magical item upon completion. Starting at level 1 just emphasizes that you're disposable lackeys. )
The book already uses the Red Wizards of Thay as an enemy faction that is out to twist everything to their own advantage. This works as-is. One set of immortal oligarchs are already taken care of.
Your patron is a woman named Syndra, who is supposed to be a member of the Harpers. I haven't got a clue what in Sam Hill the Harpers are supposed to be, so they have now been turned into the Harper East Seres Company (Or +Jacob Hurst 's Martel Company) Your friendly neighborhood expansionist venture-capitalists that have formed their own little government,who are now also immortal.
Both the Red Wizards and the Harper Company want the resurrection problem solved. Wizards want the Soulmonger for themselves, the Company doesn't yet know that it exists. The two groups loathe each other.
Elbow Grease Point 2: All about the politics
The module mentions factions, but few of them are actually important to the goings on in Chult. With rival groups of immortal oligarchs meddling in local affairs on top of local factions, that is a huge oversight.
The Harper Company
"It's just jungle now, but with some forward-thinking it transforms into something beautiful. Think about it. Rubber, cocoa, cotton, tea; plantations from horizon to horizon. Mines producing more oil and gold than we can imagine. And diamonds!"
- Wants: The end of the death curse, a return to the status quo, economic control of Chult
- Does not want: The Red Wizards gaining anything, the Merchant Princes getting too uppity and thinking they run the place.
- Is currently: Throwing poor people at the problem until it gets solved.
The Red Wizards of Thay
"The Chultans' aspirations mean nothing; They do not play with the long view in mind. No one ever does. They just think they'll live forever all on their own."
- Wants: The Soulmonger, monopoly over resurrections and all other necromancy
- Does not want: The Harper Company gaining a foothold in Chult, Chult becoming anything more than a backwater, control of Chult for themselves, the Soulmonger destroyed.
- Is currently: Sending veteran problem-solvers to solve the problem.
The Merchant Princes
"In my grandfather's time Port Nyanzaru was a few huts and a palisade, and now we'll be selling tej in Lankhmar and Corpathium by year's end. No more "dark continent", no more superstition, just the Princes' stamps on crates of fine Chultan goods."
- Wants: To become a major player in the global market, to drag Chult kicking and screaming into the Century of the Fruitbat, to play with and then become one of the Big Boys, to get access to resurrection for the Princes.
- Does not want: Monarchist movement to gain power, news of the undead plague or death curse to spread and make them look bad.
- Is currently: Distracting the populace with dinosaur races and tej, trying to get a leg up on the Harper Company via corporate espionage.
Liara Portyr and the Flaming Fist (A mercenary subsidiary of the Harper Company)
"Fucking spear-chuckers need to learn who really runs the world."
- Wants: To "pacify" and "civilize" Chult. To get filthy rich by looting the jungle and oppressing the locals.
- Does not want: Interruptions of their making money, Chultans thinking that they should be in charge of their own country and lives.
- Is currently: Putting pressure on the Princes via pirate-permissive patrol practices
"When I'm queen, no one's going to tell me what to do. I don't need to be babied, I don't need protected - I will be queen and I will show everyone how things are supposed to be!"
- Wants: To claim her rightful position as queen of Chult, to toss out the merchant Princes and the foreigners, to return Chult to an overly-idealized former glory.
- Does not want: To remain hidden for much longer, to actually have to deal with the reality of governance.
- Is currently: Biding her time in Kir Sabal with her brother, chomping at the bit.
The common folk
"Welcome to Chult: Home of the world's friendliest mosquitoes."
- Wants: Food on the table, money in the bank, hot water, soft toilet paper, a new mosquito net, a good night's sleep, a brighter future.
- Does not want: The curse to continue, to end up with someone's boot on their neck, to completely abandon who they are in exchange for the benefits of modernity.
- Is currently: Feeding an undercurrent of worry beneath the new prosperity.
"The apes learning to talk was entertaining at first, and quickly became less so."
- Wants: To awaken the Night Serpent, to keep humans out of their jungle.
- Does not want: Acererak to stay any longer, to be betrayed by Acerak, to admit that they know that Acererak is definitely going to betray them.
- Is currently: Protecting the lost city of Omu, preparing for the return of the Night Serpent, preparing for a sudden but inevitable betrayal.
"IT WAS ME! IT WAS ME THE ENTIRE TIME! YOU ALL BOUGHT IT! FOOLS!"
- Wants: To feed souls to the Atropal, to cause widespread misery and suffering for his own amusement.
- Does not want: The game to end, to have his wants challenged by anyone or anything.
- Is currently: Overseeing the Soulmonger, planning future schemes, figuring out how to betray the Yuan-ti.
Elbow Grease Point 3: Expanding the curse
In the module proper the Soulmonger captures all the souls of the dead, blocking resurrection or phylacterization. I can only presume that captured souls are stuck in a whirling torment of pain and misery as the Atropal devours them, but we can make it more horrible. This part was originally intended as a replacement for the death curse, but it works better as an addition to it.
What if the Soulmonger wasn't just a trap for the souls of the dead, but one for the not-yet-living?
[Aside] There was a fantasy full conversion mod for Civilization 4 called Fall From Heaven II. It was and still is fantastic. In it, the primary evil religion had a shrine they could build called "Stigmata on the Unborn". While the effect in that game was powering up your units depending according to how high the Apocalypse Counter is, I always imagined it as a method of attacking rival civ's population growth. So there's the inspiration for this.[/Aside]
This effect of the death curse is still in nascent stages - we have not reached Children of Men stages yet, but the important thing is that the potential is there. The effects will spread, and gradually grow worse.
Right now, people are worried. The last few months have been filled with deformities, defects, and miscarriages most common of all. The people know this; midwives talk, after all. The Merchant Princes and their representatives deny that anything is going on; this is not a conspiracy on their part, but the simple belief that curses are just local superstition, waiting to be driven out by their modern mercantile rationality. (Of course, being filthy rich means you can blunt many of the worst side effects of the curse, for the time being.)
The reveal of this aspect of the curse should be a good OH SHIT moment for players. Not only because of the potential consequences, but for providing a moral quandary: destroying the Soulmonger ends the curse on the unborn (definitely a good thing), but further entrenches folk like the Red Wizards and the Harper Company in their power (not a good thing), thus permitting them to further meddle in the affairs of Chult to their own ends (definitely not a good thing). Beating the bad guy will not solve all your problems.
Elbow Grease Point 4: The cosmic abortion in the room
This guy reminds me of something. I just can't put my finger on it...
It's on the tip of my tongue, I swear...
Nope, can't think of it. It's probably nothing.
You can't just put a cosmic abortion in the room and not deal with the fact that there's a cosmic abortion in the room, Wizards of the Coast!
Elbow Grease Point 4: Dealing with the cosmic abortion in the room
Res minimus: Levels 5 and 6 of the Tomb should be a horrible nightmare fleshscape, as is befitting the title of this post. The giant undead fetus is right there, stone corridors aren't going to cut it any more.
Res maximus: Acererak is the Atropal's father.
Maybe he's the actual father, maybe he's only acting in loco parentis. Perhaps it was a divine miscarriage instead of a cosmic abortion. It doesn't matter: whatever you might come up with is better than "he found it on the Negative Energy Plane" and I don't want to ruin the image with explanations no one wants or needs.
[Aside] The crazy thing is that this is practically in the book as-is: they just don't give it any focus. The height and weight of your guide are super important, I guess. [/Aside]
The horrible beating heart in the middle of this campaign is that relationship. Acererak cares for the Atropal (by feeding it souls), but demonstrates a simultaneous and paradoxical apathy to it (he only appears after it has been killed, and certainly doesn't care about its state of torment). I doubt I need to spend much more time explaining how the psychopathic undead troll-DM is a really, really bad parent.
Elbow Grease Point 5: Back to the beginning
But the cool thing about that horrible beating heart (Finally! The end!) is that it calls back to Point 1:
The only applicable difference between Acererak and the Red Wizards or Harper Company is time.
Acererak has reached the point where wealth and power have fundamentally lost meaning to him because he will never run out of either - he can just build a new overly-elaborate death trap when the current project is winding down. The Red Wizards and the Harper Company aren't at that point yet, but are already stretching their abuse and exploitation muscles. The Merchant Princes aren't on the playing field just yet, but they will be if they have their way.
There will be more Acereraks in time, and Chult is set to become an Atropal of its own. The older established generation exploiting the younger so that it decays before it has grown.
Settings with commoditized resurrections aren't fun, kids. This is where you end up.
Elbow Grease Point 6: A few ifs
If the Soulmonger falls into the hands of the Red Wizards
The curse will be lifted, but they will swiftly enforce a monopoly on resurrections and undeath. The Harper Company will be cannibalized and disbanded. The Merchant Princes will be prevented from ever getting their hands on resurrection services. Chult's dawning golden age stalls out.
If the Harpers remain a major power in Chult
The Merchant Princes will be bought out - never officially, but lopsided trade agreements and crooked distribution rights (in exchange for resurrecting the leadership) will see the Princes become yet another (fantastically wealthy) subsidiary of the Harpers. The Burning Fist will become the boot stomping down on the Chultan neck. Chult will be exploited to the last acre, and by the time the people realize that they've been had, an independence movement will become much more difficult.
If Mwaxinare reclaims her throne
She will find herself surrounded by enemies on all sides and with next to no allies, vastly outmatched in resources, guile, and governing ability. An agreement might be made with the Merchant Princes or another faction, but she has no power of her own beyond lingering cultural significance. She courts disaster.
If the Soulmonger is used by the party
Difficult, but possible with a powerful enough necromancer. The scope of the curse can be modified with the device - for example, the curse on the unborn might be lifted, while the ban against resurrection might remain. Souls passing to the afterlife might be trapped en masse, individually, or set free.
You can, of course, utterly destroy both the Harper Company and the Red Wizards. Not fast enough that they won't retaliate, mind. You'll also need the cooperation of the Yuan-Ti, or their removal from the premises.
The the Soulmonger is allowed to continue its business unchanged
The curse on the unborn will spread outwards from Chult. Within a few months there will be more deaths than live births. In a year's time, there will be no live births at all.Within five years, the entire hemisphere will be effected.
If the above, and the Atropal is dead
After nine years, when the curse has spread across the entire world, the Soulmonger will finally contain enough souls to summon the Night Serpent. It will rush out like a black river from the gulfs between the stars, devour the offering, and usher in a second age of the serpent.
If the above, and the Atropal lives
If the Soulmonger is still active, the Atropal will be born into divinity in 280 days. A maelstrom of pain and hate and fear will burst forth from the deepest jungles of Chult, obliterating the Soulmonger, the Tomb, and the lost city of Omu. Most of the country's population goes mad instanteously from the initial wave of psychic torment. Everything that dies, down to bacteria, will be raised as undead. The jungle will become a landscape of perpetual rot.
If something else happens
You're clever folk, I'm sure you'll think of something