A Sea of Troubles
Tsernoth is an evil necromancer, his identity revealed to the party by way of the banshee Agatha in the woods near Conyberry.
Nothing was known about him other than that he had taken the powerful spellbook of Bowgentle and fled to Ruathym, and incomplete intelligence that he planned to make himself a vampire and use the people there to invade the human cities of the north.
Upon their arrival via the Elfmaid, it was revealed that Tsernoth had smuggled himself onto the island in the body of Xzorsh, whom he presumably murdered around the same time the party arrived into Waterdeep.
Shortly after this discovery, a small detachment of undead attacked Ruathym Town, presumably under his command.