Have you restored Mardaneus' sanity yet, by reaching the bottom level of the Aleroth catacombs? Once Mardaneus is cured, Seth should tell you the healers are needed in Rivertown. After that you need to go talk to everyone and get them to make preparations to leave, and when Mardaneus says they are ready, Seth will send you to arrange an escort while he defends the town.
