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.
Ebondeath's reign
In 631 DR, the dragon Chardansearavitriol (called Ebondeath) laid claim to the area, making his home in Uthtower and ruling all surroundings. He disappeared in 922 DR, becoming a dracolich at the behest of Strongor Bonebag, a priest of Myrkul. The Ebondeath Sect of the Cult of the Dragon began worshiping him at a mausoleum created at the Uthtower.
In 1202 DR, under the Eye of Myrkul, Ebondeath's body disintegrated into dust, due to the god Myrkul's influence, although his spirit was still tethered to his remains. Worshipers from the Ebondeath Sect traveled to the Mere of Dead Men to see the remains and the sect grew. However, the Ebondeath Sect collapsed around 300 years later in 1358 DR when Myrkul was destroyed. Sect members' powerful rings of Myrkul were lost to the swamp monsters.
Rise of the undead
When Myrkul's avatar was destroyed over the Sea of Swords, fragments of his bone dust were carried north and entered the waters of the Mere. This dust animated some of the drowned corpses of the orcs and people of Uthtower into a variety of lesser undead creatures. They displaced many of the living residents of the swamp, though the undead lacked any kind of direction, having no greater undead or necromancer to lead them.
As the undead began to make an appearance, members of the Cult of the Dragon cell in Leilon began searching for the mausoleum.