Kernel of an idea that came to me while driving. I was listening to a podcast (Make Me Smart) and they were talking about the move by the right wing from Facebook to apps like Parler. One of the hosts mentioned that someone had pronounced it like parley instead of parlor and the thought of pirates immediately leapt into mind.
Why would pirates respect a code of conduct like the law of parley (as seen in Pirates of the Caribbean)? Perhaps they are magically bound to do so.
I thought of a large, literally black sea. It could be in the overworld or contained in a vast underground cavern. Any who try to enter the waters or even sail upon the black sea are consumed by it unless they first make a pact to bind themselves. Those who are bound to the black sea must remain within a certain distance of its waters but they are otherwise safe from its deadly consumption (able to swim and sail). This binding includes certain idiosyncrasies, one of which is the law of parley (the rest to be determined later; they will likely include certain weird/faerie things that make little sense to us but serve an unknown/unfathomable [see what I did there] purpose). If one breaks the laws of the black sea, various ill fates may befall them (black tentacles from the unseen depths drag you down; a host of bloated corpses [previous unbound who met their demise] clamber upon your ship or even onto the nearby shore to tear you apart; etc.).
Why would one want to bind themselves to the black sea? Because it is filled with islands brimming with rare resources and outright treasure. Despite its dangers, there are settlements on its shores where one could live it up after acquiring riches from its dark waters. Many exotic plants, animals, and materials are to be found in the black sea and merchants hawk them to traders who visit the nearby cities and town.
Some ideas can be found in an old post about a Pirates of the Sunless Sea idea by Phindar (http://eldriciv.blogspot.com/2008/05/underdark-locations.html). A somewhat similar but separate idea is the Black Pudding Lake (https://eldriciv.blogspot.com/2017/09/black-pudding-lake.html). And perhaps at the center of the black sea lies a lost island (http://eldriciv.blogspot.com/2008/05/lost-island.html).
