Scales of War
According to Modra, the brass key you found in the previous adventure is a one-way activation key for the transfer portals beneath the Happy Beggar. Modra had lent the brass key to Iranda, the dark creeper liaison between Modra’s arms-dealing forces and Tusk’s orc armies whom you slew in the Vents, leaving Modra only in possession of the return key from the Shadowfell. When Sarshan discovered that Modra had gone behind his back to negotiate the deal with Tusk, and that he had moreover lost an activation key, he was livid with anger. He demoted Modra, and promoted Skuld to take his place. He gave Skuld a one-way key in order to carry out smuggling and slaving operations in the Shadowfell, but wary of any further evidence of his activities coming to light, he entrusted the return key to Thannu, the captain of his tower. In order to return to the mortal plane, you must slay Thannu and retrieve the return key.