Lahnis's faith in Szass Tam had begun to flag; he was no longer certain that Tam's plot was the best for Thay. He was intrigued by the Thayan Resurrection rebel group but never joined them. He was too scared that Tam's faithful controlled him.
Lahnis was a Red wizard assigned to the secret complex of Doomvault under Thaymount. He was tasked with training the Thayan warriors there.
When, in 1486 DR, the heroes of Daggerford arrived, Lahnis at first thought they were troublemaker agents sent by Szass Tam to test his loyalty. Eventually, the heroes persuaded him to defect to the Thayan Resurrection and Lahnis helped defeat his former subordinates. He later told the adventurers about the dungeon.