The Mere of Dead Men (or Merdelain, meaning "Slow Marching Court" in Elvish) is a cold saltwater swamp roughly 100 miles long by 30 miles wide along the shore of the Sword Coast North. It is a desolate place full of monsters and few members of the civilized races. The swamp constantly grows over time, swallowing homes and property located close to its boundaries. It is believed that the eastern boundary of the Mere is 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 was created in 615 DR. After the armies of Phalorm were overrun by the Horde of the Wastes in 614 DR, the few that had rallied made their stand at Iniarv's Tower, a tower that had once belonged to Iniarv, the former Mage Royal of Uthtower. Unbeknownst to all, Iniarv had retreated into the tower's crypts to become a lich. He was awakened by the orcs and enraged by the commotion. King Uth VII begged him for help, calling upon Iniarv's ancient alliance with Uthtower. Iniarv acquiesced, calling on the ocean to flood the entire area, killing all goblinoids and humanoids alike. When the waters receded, all that was left was a saltwater swamp. The land was never permanently resettled, although there were a few attempts that were thwarted by the wetlands and strange monsters released by Iniarv.