So, Those Things That Come Out of The Jungle
The Buta-Kala are ogre-sized demonic humanoid figures. Their distinguishing features are bulging eyes, severely overgrown eyebrows/mustaches, elongated arms (like orangutans), long claw-like fingernails, and grotesque, curving fangs. Baaaaaaackground: These are the prominent villagers of the ruined city that long ago forsook their now-forgotten god and as a punishment were turned into these evil spirits. Their wild cries sound like some kind of language pushed to the edge of bestiality.
If you're lazy, the Buta-Kala can be approximated by substituting Ogre Magi stats, with the caveat that they will launch into an immediate frenzied attack rather than any sort of carefully thought out tactics and replacing weapon attacks with 2 1D6-damage claw swipes. The Buta-Kala will attempt to carry off any player who falls unconscious during the battle. There will be 4-6 of them and while tremendously strong, they aren't very smart, leaving room for clever parties to avoid them/lay traps/etc.
The Buta-Kala will retreat either if 2 or more of them are killed or if they are able to carry off an unconscious party member.
The aromatic oil and incense can be used to repulse the Buta-Kala -- for each container of oil tipped on or incense cone burned on the old altar stone, the Buta-Kala will stay in the jungle for the night -- the incense has to stay burning, the oil just has to be poured on the stone. If they choose to retain the items, the oil is worth 200gp per container and the incense 50gp per cone to an interested buyer.
The two kris blades are blessed blades that can damage evil spirits. For the purposes of this scenario in my particular campaign, this means that they an extra D6 on top of the Strength + Sword roll against the appropriate enemies -- they still do a bog-standard D6 worth of damage. You could just make them +1 weapons or +1 weapons vs. spirits or whatever works best for you.
Following the Buta-Kala OR What's in the Jungle OR Where'd the City Get Drinking Water
Any sort of expedition to find the Buta-Kala after they've run off into the jungle (more likely if they're carrying somebody) or into the jungle itself during the daytime will reveal that the jungle immediately surrounding the ruins is scattered with coral jewelry, volcanic glass knifes and other detritus, including a handful of heavily rusted metal blades.
East of the ruins an experienced tracker or a determined search will reveal what might have been a fairly broad and ancient path, not almost completely overgrown and forgotten. Following the path will lead to a small freshwater dribble. The water will have a foul stench to it and any party members drinking from it will become violently ill, vomiting up the contents of their stomach and contracting a fever that will keep them indisposed for the next 8-24 hours (Constitution/Vitality checks every 8 hours, a successful result means the illness ends).
If the creek is followed, it will slowly grow larger until it is a respectable gush coming out of a 5' by 10' crack in a rocky outcropping near the center of the island. The smell will be more powerful and a small ground-hugging mist can be observed lapping at the trees as it flows out of the cave along with the water. Entering the cave will reveal a 30' wide circular chamber with a misty and corrupted volcanic spring littered with an incredible amount of bones (enough to account for a hundred or more people) and an old lady puttering over a camp fire. If a party member has been abducted, they will be lying in a stupor near the fire, stripped nude and with various colorful patterns drawn on them.
Of course, the old lady is not what she appears and is actually a Rangda.
The Rangda is a hag-like creature with similar physical stats to a standard D&D hag if that's how you roll. Her big magic attack is to create a magical field 30' around her that will cause all inside to make a Saving throw vs. Death Magic (or Vitality vs. Willpower in my case) or immediately start attacking themselves with their own weapons. Any character doing so will automatically hit themselves once per round, rolling damage normally. They will continue to do so until they die/fall unconscious -- they can be broken out of these trance by being slapped/physically restrained, which will allow a reroll of the save for every round they're trying to be roused or automatically rescued by the use of an appropriate spell (Protection From Evil, etc.). Those not doing so will be attacked physically by the Rangda, who strikes with two 1D4 claw rakes as well as any further magic attacks that you would find fun/appropriate.
If the party is carrying burning incense from the ruins, the Rangda's suicide magic is ineffective, although she will do her best to put the incense out. This also goes for if she is doused with the aromatic oil. Any wielders of the two kris blades will receive a significant bonus to their save and instead of attacking themselves will go into a meditative state until they make their save.
If the Rangda is killed, the mist and smell disappear, along with the toxic effects of drinking from the spring, which is now clean. The bones will remain and some investigation of the spring's pool will reveal some ornate jewelry and scattered coins as well as the possibility of minor magical items (again, left to the discretion of the DM).
Most importantly, the Rangda will no longer be able to summon the Buta-Kala and future nights spent on the island pass peaceably.
Getting Off the Island
The hanging portal is the most obvious way to get off the island, as, if you're a kind DM, it will lead straight back to the portal that the party entered originally (although in my campaign, the odds are that Zorlac has tracked the group to the portal and has prepared a welcoming party for them). If you're not feeling so generous, the portal may instead lead somewhere else -- one potentially hilarious (for the DM) idea would have the destination of the portal change and have it based on astronomical events (in the current world/plane, of course, which would mean that the party would have no idea what they were).
The caveat here is that like the rest of the ruins, the portal is in fairly bad shape. Any sort of manhandling or rough activity on or around the portal stands a good chance of damaging the structure, which should then set off a chain reaction of the portal itself falling from its chains and smashing on the floor. Should the party be working with Corsica Nyne and it looks like they're seriously considering returning to Fenrecz without the book, she will intentionally smash one of the chains, causing the aforementioned destruction of the portal.
There are enough trees and lianas on the island to Kon-Tiki a raft together, with the main problem being the construction of a sail. Given enough clothing/bedding/tents donated to the process and somebody willing to do some serious seamstress work, a small sail could be created from random bits of cloth. With a party of a half-dozen able-bodied workers and somebody with at least a small bit of sailing/boating experience (somebody better have an axe as well), a minimally sea-worthy raft could be constructed in one day (if you started in the morning and you wouldn't have enough light to push off when you were done, unless you were particularly foolhardy).
The main obstacle to getting the raft off the island will be getting it through the barrier reef without crashing into the reef itself. There are some pretty large gaps in the reef; however, odds are that the raft is going to be pretty clumsy to maneuver and successfully getting it through will be tricky -- if any sail is lowered and the group relies entirely on paddling/using a sweep, it should be doable if they pick the right gap. If the party manages to convince the sea-slugmen to help, it will be an automatic success.
Another option will present itself if the party successfully engage the sea-slugmen in a conversation about the island and its surroundings. Information of note:
- Ships that pass by the island almost always appear and disappear in the same two directions (the boat that came most recently came and disappeared from one of these)
- There's a number of wrecks lying on the seafloor around the island. Most of these are just some bits of the hull; however, there's a small dhow-style boat that hit the reef a couple months ago. There's a big hole in it, the rest of the ship appears to be in good shape.
- There's a group of crabmen that live in one of the outer reef structures. While they don't generally bother the sea-slugmen, they will come and eat their eggs and occasionally attack the sea-slugmen themselves when food is scare.
Although the sea-slugmen aren't smart enough to put it together themselves, if the party suggests it, they will readily agree to bring back the dhow to the island in exchange for help dealing with the crabmen. Fortuitously, they do raise a particular kind of sea-sponge that, when a fist-sized chunk is eaten, will enable the imbiber to breathe water for the next two hours. They will provide the party with 12-18 of these chunks, although they will dry out and become useless if left outside of water for more than 5 minutes and will start to break down after a week even when left in water.
The crabmen make shelter in a small coral grotto and there are 8 of them -- they don't have much of value to outsiders aside from their shells and claws, which could be recycled by the party as armor and weapons. They'll be armed with coral-tipped spears and their claws. To get inside the grotto, the party will either have to weigh themselves down or be proficient swimmers.
When the sea-slugmen bring the dhow back to the island, it will be in decent condition, albeit with a 2'-wide hole near the front of the hull. Anybody with shipbuilding or carpentry skills will be able to patch it using wood from the island in a days work. The hole will still leak slightly unless the party is able to come up with some kind of sealant; however, diligent bailing will make the boat seaworthy again. Perhaps most important, there will be two large chests in boat that when opened, will reveal most importantly, a set of sails (the originals would need heavy repairs) and trade goods (some amount of coin, jewelry, valuables -- roll yer own!).
The closest populated island is just under a week's worth of sailing away, with the other direction being 10 days. Both of these are with decent winds and will be easier in the dhow, which can tack (it is likely that the raft cannot).