Morgajel goes underground with this release. The Player's Guide for The Ronden Marr is now available for purchase. No matter what system you want to use, you can add this book to your campaign, as it's system-free. There are a lot of deep and dark places underground. There's lot of really interesting flora and fauna to find... some might even not try and eat you!
More than just a bunch of ideas for things living underground, The Ronden Marr is a rather unique setting. The surface world was burned away during The Calamity. Now, everyone left is living in one formerly-dwarven kingdom. Obviously, all these races suddenly found elbow-to-elbow with each other is going to cause issues. Can your players help diffuse all sorts of misunderstandings and conflicts that will occur there? You'd better hope so, since there's not many other places you can escape to.