Oakun wants you to take the Dread Relic to the Writhing Mound.  Once there, kill Auchenai Death-Speakers and Auchenai Doomsayers to collect 20 Doom Skulls.  Then find the Writhing Mound Summoning Circle and use the Dread Relic to summon and destroy Teribus the Cursed.  Return to Oakun when the deed is done.