Mere of Dead Men

The Mere of Dead Men was a cold saltwater swamp roughly 100 miles (160 km) long by 30 miles (48 km) wide along the shore. It was a desolate place full of monsters and few members of the civilized races. The swamp constantly grew over time, swallowing homes and property located close to its boundaries. It was believed that the eastern boundary of the Mere was magically bound to the High Road (perhaps via a wish spell from the Mere's creator), since attempts to reroute the road were futile and only caused the expansion of the swamp. The Mere itself was full of trees, vines, quicksand, and hidden islands, and it was generally covered in fog, making visibility very poor. The bones of fallen creatures were clearly visible throughout the Mere. The water was deep enough that it could be navigated on a flat-bottomed boat, but the dark water and hidden obstructions made that choice dangerous.
Inside the Mere were the flooded ruins of a number of castles and settlements. Many of the castles had strange wards that would either strengthen certain magic or negate it.
The name "Mere of Dead Men" referred to the thousands of dwarves, elves, and humans of the Fallen Kingdom who were slain here during the invasion of an orc army.

Geography

The Mere was located along the Sword Coast on the Sea of Swords between Neverwinter and Waterdeep. The High Road skirted its eastern edges on its way through the foothills of the Sword Mountains to the east. The nearest significant settlement was Leilon to the north.

Ecosystem

A variety of creatures were found in the Mere, including bullywugs and lizardfolk; sivs; aballins, behirs, flying fangs, giant frogs and giant toads, gibbering mouthers, giant leeches, hydras, meazels, monitor lizards, nyths, scrags, shambling mounds, snakes, and will-o'-wisps. A pair of black dragons (known as a single dragon, Voaraghamanthar) made their homes in the swamp. Doppelgangers dwelt in some of the ruined settlements, posing as humans in order to lure unsuspecting travelers to their doom. Trolls also once lived in the Mere.
After the Time of Troubles and the death of Myrkul (1358 DR), the taint of the dead god infected the remains of the dead in the Mere, causing them to turn into undead ghouls, skeletons, and zombies that attacked travelers on sight.

Legends spoke of liches, a penanggalan, sea zombies, darktentacles, yuan-ti slavers, and other monsters, but their presence was not confirmed