This is the inaugural Pathfinder campaign for my current group, which I inherited after another GM who had to split his massive 10-player party into a more manageable size. This gave me the following parameter:
The party has been set adrift at sea by pirates, without any resources, on a sinking lifeboat.
That primer led me to conceive Tecal, the lost island island empire. The general premise that places Tecal in the context of Golarion is that after Earthfall, an offshoot of the Thalassian empire established itself on an island between the continent of Garund and the Azlant ruins.
The theme for this campaign is jungle horror. This is Heart Of Darkness style, with eldritch elements thrown in, making for an oppressive, suspenseful adventure.
They were an ocean-faring culture, skilled in arcane arts and trading. The source of their power was a runewell nestled in the volcanic heart of their island, and they abused it without inhibition. They adorned their abuse with a caricature of belief, claiming to be the clergy of the Peacock Spirit. At the height of their strength, their sorcerers triggered a volcanic eruption that destroyed their capital city and killed most of their ruling class. They were slavers, but they had relied on their arcane trinkets to enforce their authority. With that power gone, the slave uprising that followed uprooted the empire overnight.
All they left behind was a cursed island, still protected by immensely powerful magic that destroyed any ship that wandered too close to its shores.
The island of Tecal, cursed as it is, has been infested by undead for centuries, and almost every living thing has been twisted, distorted by that same energy. It’s divided into 4 zones. Each zone has a different environment and set of encounters that relate to the central theme. The plot supports the theme by making it a story about human arrogance and sacrifice. It is full of dangers, living and dead, active and passive, including cursed objects and haunts. Tecal’s history is an infected wound on the island that refuses to heal. New arrivals seem intent on making the trauma worse.