Sixth House Base

Caius Cosades in Balmora, Caius Cosades' House will warn you that the next assignment is very tough and advises you to make sure you're ready for it. Apparently, a Legion patrol stumbled across a Sixth House base near Gnaar Mok. Your orders are to find the base and kill the priest Dagoth Gares.

Go to Gnaar Mok, talk to just about anyone to find out information on the Ilunibi cavern (disposition 50+). You find out that the caverns are located on the north end of the island, on Khartag Point. You should have Khartag Point marked on your map when you ask about it, just a little northwest of the village. You should find the caverns named as Ilunibi, Carcass of the Saint.

Once you get into the final section of the caverns, Ilunibi, Soul's Rattle, you should encounter Dagoth Gares, a unique Ash Ghoul. He will initiate conversation with you and you should try and get as much information as you can before you exit: he can tell you some pretty interesting things about the history of Nerevar and Dagoth Ur, at least as seen from Dagoth's insane corrupted perspective. Once you exit the conversation he will immediately attack you. Gares is tough, casts spells, and hits pretty hard, but doesn't take too many big hits before he goes down. When he dies, however, he curses you with Corprus disease.

Head back to Caius in Balmora. He is concerned that you have corprus, but has been gathering some information of his own and has found someone who is doing research into Corprus Disease with a view to finding a cure: the Telvanni wizard, Divayth Fyr. He suggests you visit Fyr on the grounds that if he can't help you, nobody else can.