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.

The original Underking was Ysmir Wulfharth, a nordic king who was a shezarrine, and a dragonborn, who died, and was brought back to life as the semi-immortal "Underking" by Shor. Wulfharth could die, but he would just be blasted to ash then reform his body years later.

When Wulfharth was finally killed, by having his soul stolen by Tiber Septim, and Tiber's battlemage Zurin Arctus, Wulfharth's heart flew out of his body, and blasted a hole in Zurin Arctus's chest, which basically transffered the powers and title of the Underking to Zurin Arctus, as Arctus rose from the dead, as Wulfharth had in the past, and started using the name Underking himself.

There is a whole lot of background stuff about how Tiber, Zurin, and Wulfharth, were all actually three separate parts of the oversoul of Talos, so they are kinda all the same person in spirit, but not physically, but that is a whole other bag of worms.

And no, there is no underground civilization, the name Underking refers to Shor, the ruler of the underworld, aka Sovngarde. Wulfharth was a Shezarrine, an avatar of Shezarr, which is the Imperial's name for Lorkhan, aka Shor, as he is known to the Nords. Shor is the king/ruler of Sovngarde, so his avatar would be "the Underking", the king of the underworld/Sovngarde. Though the title has only been used by those shezarrines who have died then come back to life, aka Wulfharth and Zurin.