(Okay, so I need to start this review off with the disclosure that I was one of the people on Google+ providing feedback and suggestions to Jesse as he crafted this nice bit of work. He kindly included me in the credits with the other helpers. Although I got a pdf of the final product free from the author I bought a publication copy anyway to express my support for the project.)
Ronden Marr is a campaign setting for a fantasy RPG. It is a generic setting not tied to any specific game system. This first publication is the Player's Guide, a 77-page pdf available at Drivethrurpg.com. Even though it is a player's guide it provides a complete enough description of the central setting (the underground city of Ronden Marr) that a GM could base a game on it. Additional core documents are planned, as well as adventures and some D&D 5th Edition specific rules; there is a Patreon available if you want to help the project along.
The setting is the underground city of Ronden Marr. Originally it was a dwarf city but a devastating calamity rendered the surface of the world too hot to inhabit. Refugees of many other races were allowed in. As far as its current inhabitants know this overcrowded city is the only remaining inhabited place left in the world. The city has had a bloody history since then, with uprisings, genocide, enslavements, and now a brutally enforced peace. Below the city is The Undercavern, a deep, dark place filled with dangerous creatures but much potential. (I think Ronden Marr can be run as-is out of the box just fine or you could plop it into an isolated corner of your existing world with some adaptation.)
The player's guide posits that the characters will either be "consultants" who serve as oath-sworn troubleshooters for the dwarf overlords or be "freelancers" who make a living doing odd jobs which often require not asking too many questions. There are plenty of adventure hooks to keep the players busy no matter which path they take.
It's an interesting campaign world in one compact, well-written pdf. At just $1.99 you can't go too wrong getting yourself a copy.