Ruleset 5e Dungeons and Dragons, Dungeon Crawl Classics and ICRPG with that strong influence of Yoon-Suin and a hint of horror.
3 different rules sets running in the same campaign setting.
The City of Sensay, formally known as the Yellow City, has a diverse culture, beautiful architecture, all are treated fairly, or at least that is how I would brand this place for tourists. The diversity is apparent, but even a quick visit and you would pick up on the strict social order that is in no way balanced or fair.
It never fails to impress a visitor to the Yellow City that its citizens are by turns the wealthiest, most refined, and most educated people in all the world, yet at the same time capable of the most malicious cruelties and licentious depravities. Like all those whose societies are ancient and rich, they are also cynical and filled with restless melancholy.
The most singular feature, which strikes any visitor upon arrival, is the strict hierarchical stratification, which all inhabitants obey without question.
In the highest strata are the slug-people..
Cursed Kingdom is a homebrewed campaign that changes quicker than a tiefling who learned a disguise self cantrip. This campaign is compliant with OGL because it is not intended for profit or resale. I will try to mention when I take pieces directly from other works.
The main inspiration will be from Yoon-Suin, by David McGrogan, with help from all the tips, tricks and advice readily available these days. Game style will be as suggested in Yoon-Suin with original work added to the random tables as well as works from 5e D&D and even The Dungeon Alphabet, by Michael Curtis.
Essentially travel will be a hex crawl with limited major locations noted, mazes will follow a basic mechanic (thank you Hankrin Ferinale at Runehammer Games), and more expanded random tables than you can imagine. This game should give that sandbox feel while being so far from railroading you will feel like you are in the stone age.