Seas of Sand

From the introduction:

By day, the sands are liquid. People sink and ships sail. By night, the sands are solid. People can walk and ships are stuck.
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Seas of Sand is a toolbox setting guide. It details the Seas, its people, its stuff, and its wonders. It’s also a whole bunch of procedures, advice, and content for you to use in your game. There’s lots of content here, but no plotline and—other than examples—no fixed locations or events. There’s no history to memorize, no lore to wade through, and no canon to adhere to. Seas can slot into an existing game just as well as form the basis of a new one.