The Sargussum is a region of the Great Ocean in the Straits of Tlaxot off the coast of Lustria that is riddled by a thick seaweed of the same name that often traps any sea-going vessel that seeks to move through it.
The waters of the Straits of Tlaxot are home to a type of seaweed also called the sargassum that can trap ships seeking to move through it forever, unless they find a path through. Vines thick as a ship's cable grow from the ocean floor to spread along the surface, where they form tangled mats the size of small islands. Exposed to the heat of the sun, when portions of the seaweed die they slowly rot and emit a charnel stench strong enough to nauseate unaccustomed mortals. Vast flocks of seabirds swirl overhead, pecking at the corpses floating among the morass.[1a]
The Sargassum is the "port" of the Vampire Lord Luthor Harkon's Zombie Pirate Dread Fleet. As far as the eye can see, dead ship after dead ship floats mired in the seaweed, many overgrown with its vines. Some are recent, some so old that they are little more than rotted hulks. Ships of all nations rest here -- cogs, carracks, and galleons of the Old World, great High Elven ships from Ulthuan, and even enormous junks from Grand Cathay.[1a]
All are still, no wind filling their sails. Passing sailors occasionally spot activity aboard them and then quickly avert their eyes when they realize the crews are Undead, going about a seaman's duties in a grim charade of life. When the ships sail on Harkon's orders, however, the weeds release their grip and, wind or not, the Dread Fleet sets off to harry the living.[1a]
Ships traversing the Straits of Tlaxot must be careful or they will become entangled in the sargassum. Escape requires many hours of chopping and cutting the seaweed, although several hours after becoming caught, a "pilot" -- usually a Ghoul, sometimes a Human -- arrives from the Dread Fleet offering the stranded party passage. The fee is five gold coins per crew member, or one fresh corpse. If declined, a boarding party of Undead attack the mired ship that night. If they succeed, it and its crew become part of Harkon's fleet. If they fail, the ship is allowed to pass.[1a]