I know the Underking from Daggerfall where he is described as the mage in service to the first emperor of tamriel. He is also mentioned as the king of the Underworld. Morrowind added some lore regarding the underking and possibly other tes games. So my question is what exactly is know from all tes games about the underking? Also is the underworld a figure of speech of an hidden society or is there a underground civilization in the same style as Jules Verne books?
PS: In case someone from The Imperial Library is reading, that site as a series of fan made articles about the story of each game which are great. It would be awesome if you also write a series of biographies for the most interesting characters with links to book sources.