From the Morrowind Launcher, select Data Files and check the box
next to the "Siege at Firemoth.esp" file.
The island fortress of Firemoth was taken by the skeleton army of Grurn years ago. It's time to take it back. Sellus Gravius, Captain of the Seyda Neen Census and Excise Office, hires you to lead a band of adventurers to the Fort.