In the distant past, during the time of the Great Cataclysm, a large area of land collapsed, sending most civilized areas crashing into ruins. The Plax and Shoon Rivers poured down over the ensuing escarpment, flooding a depression which became the Black Marshes and Lake Hlan.
A sorceror of notorious power known as Thalanaq arrived in the area and erected a stone tower on the eastern shore of Lake Hlan. His foul expirements and magicks turned the viscuous mud and waters of the marsh black, accompanied by the foul odor of brimstone. He also unleashed foul beasts and malformed creations to roam the stinking marshes to keep away intruders.
In time, Thalanaq's demon patron, Ysidruul instructed Thalanaq to build a city and attract others to worship it. Thalanaq began the task without question. He went forth and gathered many men and women who were barely surviving in the surrounding lands. He told them he could provide food and protection in exchange for their labor to build the city and to pay homage to Ysidruul. And so over many decades, the city of Thalan was built around the site where his tower stood. A temple to Ysidruul was built and Thalanaq's tower was expanded and became what is known as the High Tower of the Watchers in modern times. The stone of the tower is carved with the motifs of many eyes, many of which move and follow passersby. Thalanaq took on apprentices at Ysidruul's urgings to teach them sorcery. Benath the city, a network of sewage tunnels were built with outflows feeding into Lake Hlan.
Little did anyone know that Ysidruul was in fact an Aboleth who had managed to dominate Thalanaq and had him build the city so as to expand it's own base of power. With more people gathered in one place, Ysidruul was guaranteed a constant food source and slaves to it's will. All would have been weel except that one of Thalanaq's apprentices was exceptionally more strong-willed and discovered the threat. He managed to break some of the other Watcher's from Ysidruul's influence, but this alerted Ysidruul and he sent Thalanaq to deal with the upstarts. A great sorcerous battle took place and many apprentices were killed, but Thalanaq was finally brought low and slain. The temple to Ysidruul was torn down. The remaining Watchers discovered a spell that drove the Aboleth from the city and into Lake Hlan. That was over 100 years ago and the modern-day Watchers now keep a vigilant watch over the lake, performing rites to keep Ysidruul from returning.
Thalan today is in decline and a corrupt place, seeking it's hedonistic pleasures, aloof of the plight of the lower-class. The city is governed by the Mallisartu family, once a powerful merchant family that rose itself up to noble status. Wealth that is now squandered away on pleasures of the mind and flesh, sequestered away in their palatial homes. The city is mostly lawless and the city guards cater to the many factions within the city. Anything can be bought or sold for the right price. Thalan is a city focused on pleasure. Whether that is brought about through drugs, alchohol, love-slaves, or the gladitorial combats in the local arena. However very few can afford such luxuries and there is a large lower class that are exploited by the few in power.
Outside the walls of Thalan, the marshes and surrounding lands are a dangerous place. However, to the desperate few brave enough to seek their fortunes, there are old ruins to explore for riches. Many of the rich in Thalan would pay greatly for the exotic hallucinagetic plants found only in the steamy Jungles of Kesh or the Fungal March. Also exotic creatures are sought after to be captured for the arena games in Thalan as well.
Here is the link to a PDF file for the One-Page Wilderness Template detailing a map, hex keys and wilderness encounter tables: Black Marshes of Thalanaq