Over many years I've been working on ideas for a Sword & Sorcery Bronze Age setting, constantly revising and polishing. I've now started running my third campaign using the latest version of this world, as I wanted to run an Isle of Dread campaign and just happened to have a well outlined world at hand that I get to use far too infrequently. I've written quite a lot of general stuff about the world in the past, but now I am turning the Isle of Dread into a fully detailed sandbox with specific locations, NPCs, and local history.

At the heart of this campaign and the sandbox is the famous old module The Isle of Dread (by David Cook), with a good helping from Dwellers of the Forbidden City (by David Cook), and I've used the equally famed Against the Cult of the Reptile God as a starting adventure on the mainland to get the player introduced to the setting and give them some hook for their further explorations.
At the heart of the Isle of Dread is big crater with an ancient lost city, but it is pretty disappointing in its scope, so I am replacing it with the Forbidden City. Other old modules I always wanted to use, and which all found great spots on the isle are The Forgotten Temple of Tharizdun, The Hidden Shrine of Tamoachan (edited by David Cook). And within the Forbidden City, I will be using The Temple of Death (by David Cook). I'm also including the mysterious monastery from Masters of the Desert Nomads (by David Cook), the upper levels of The Forge of Fury, the basic framework of Torrents of Dread, and possibly parts of Keep on the Shadowfell. (And I also managed to squeeze in Escape From Meenlock Prison as a stop along the sea journey to the isle.) That's about 90% of all D&D adventures I ever wanted to run. I am running this using D&D 5th Edition, though I am more or less using the rules for dungeon and wilderness exploration from the Expert Set (by David Cook.) I love the man's style of D&D.

Spoiler: Maps
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Inhabited places red, other sites orange.


The Backstory of Inixon
In an unknown past age, an ancient civilization known as the Glass Makers build four of their great towers of green glass on Inixon in the Southern Sea (9, 16, 25, 30). Each of these towers reached deep into the earth until they came to a chamber just at sea level, that holds a great black sphere, about 4 meters across. The towers are set in such a way that the great volcano in the center of the island is exactly at the center between them, about 72 miles from each tower. The four spheres are all conduits to the black and cold darkness of the Void Beyond the Stars, though why the Glass Makers created them remains unknown.

At some point the island was apparently inhabited by the Rock Carvers, as there are a few ruins in the mountains that have all the marks of their architecture. But their presence on the island appears to be limited and perhaps only of short duration.

Long after the Glass Makers had disappeared from the face of the earth, the Southern Islands became part of the great naga empires. Naga sorcerers discovered the towers and the spheres and studied them for centuries, building The Forbidden City inside the crater in the center of the island (35), as well as Tamoachan (15) in the Southwest and another city in the Southeast (1). Like with the four towers, a very deep shaft goes deep into the earth underneath the city. These tunnels are filled with rainwater, but also connect to the sea and are the domain of an ancient aboleth. The naga managed to capture the aboleth and tried to make him tell them the true nature and purpose of the spheres, but they never learned anything from it.

When the world turned colder and the naga had to abandon most of their empires to retreat south, the naga on Inixon turned to the power of the four spheres. They were convinced that with their power, they would be able to create the Black Sun above the Forbidden City to provide them with the warmth that the orange Sun no longer gave them. The attempt failed spectacularly, causing the southwestern sphere (16) to explode and tear away the entire southwestern peninsula. The southeastern sphere (9) exploded less spectacularly, only creating the great lake and toppling the tower that stood above it. Unnatural cold spread over the island and only the priests at the fire temple between the two volcanoes (14) near Tamoachan survived for a few month longer before they died as well. The aboleth remained trapped in a pool, unable to return to the flooded passages by itself, and so it simply waited and slept through the following ages.
(Also, something blew up the southeastern peninsula, leaving behind only small islands in a ring around the main explosion. The great wall seperating the area from the main part of the island was not keep the monsters from the jungles to travel south, but to keep something from the crater to reach the main island. No real clue what that was yet, though.)

Almost a thousand years later, a great typhoon completely destroyed several of the Southern Islands and some refugees ended up on what was left of the southeastern penisula. A few decades earlier, a group of disciples of the moon goddess Temis came to the island and created a monastery in the Rock Carver ruins high up in the mountains (17), seeking solitude in the most remote place in the known world. Over time they became corrupted and turned away from seeking enlightenment in the Moon and instead started to seek out the mysteries of the Black Sun.

Eventually an age of relentless storms brought the approaching end of the humanoid civilizations on the mainland and the naga in the jungles beyond the Southern Sea are sensing a time to reclaim their lost empires in the north. A naga sorceress named Yinethi remembered the stories of Inixon and led an expedition to the Isle of Dread. They made a deal with the pirates who had a hidden lair on a nearby island (5) and set up a camp in the bay that was created by the exploding sphere. The pirates were not as silent about the deal as they thought they were, and word eventually reached the Sorcerer Lords of Ven Marhend. They send a sorcerer to find Inixon and claim the ancient power in the island for them, and with the help of a mercenary company he managed to find the old paths to the Forbidden City before the naga did. The naga arrived there a few month later and set up their camp in the great palace, while the sorcerer and his mercenaries have fortified one of the old mansions as their base. Mostly they are avoiding direct confrontation, but neither has any intention to share the power of the island. Meanwhile the naga are trying to find the pool of the aboleth, hoping that they can make it reveal its secrets to them.

Yinethi send her servant Szar to the mainland with the pirates, to establish a base from where they could learn what had changed in the thousand years they were gone, and to capture slaves in a place far away that would not draw attention to the Isle of Dread. Of course, Szar screwed it up and the Cult of the Reptile God was discovered and destroyed. And prisoners told their rescuers that the cult leaders had been waiting for a pirate ship to take them place named Inixon.

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The way all this came together was really cool. I started the whole concept knowing only that I wanted to use The Isle of Dread, Dwellers of the Forbidden City, and Against the Cult of the Reptile God as a serpentmen campaign. I also always wanted to try running The Forgotten Temple of Tharizdun and put it somewhere in the most remote mountains on the island so it would feel well hidden if the players find it. Putting two glass towers into the two swampy areas was a spontaneous idea, and I did not have any plans for what to do with them yet. I just thought green glass towers are cool. And only this week I noticed that on my map, The Forbidden City, the Forgotten Temple, and the two glass towers form three arms of a giant X. A fourth arm would point right at the naga landing site. Except for one of the towers, I have to move the locations by only one hex to get a perfect X.
The big attraction of the Forgotten Temple is a magical black sphere deep below the temple, whose exact nature and function is left unspecified. I had an idea to have a similar but broken sphere in the Keep on the Shadowfell and put it somewhere near the starting villages, but noticing the extremely conspicuous location at the naga landing site, I moved it there instead. And having a broken sphere right where the mountains reach a great bay obviously invited some kind of massive explosion.
In one of my earlier campaigns that never got very far, I wanted to use Dwellers of the Forbidden City and its aboleth as a ripoff of the Mass Effect planet Feros. Which has an ancient all knowing creature hibernating below a ruined city and villains come to steal its knowledge, so it uses its psychic power to enslave human explorers to defend itself. With the villains getting played by serpentmen. I want to use this idea again, which is how the aboleth comes into the picture.

Since this is for a sandbox campaign of undetermined length, most of this might never actually come up. But a big part of sandboxes is that the players can explore the world and learn some of its secrets and histories like in Dark Souls. I also should have some idea what happens when the players find the naga in the Forbidden City and decide to not just kill them all immediately, or they want to but are unable to. That's what this history is for.
I plan to write up my idea for all 42 sites in this sandbox and whatever additional sites I might add to it later, and suggestions are always welcome.