Well, I won't pretend to know much, but I do have access to the wiki, which almost tells who he is, in case you haven't checked.
http://www.uesp.net/wiki/Lore:Underking is usually regarded as one of three people: http://www.uesp.net/wiki/Lore:Wulfharth, http://www.uesp.net/wiki/Lore:Zurin_Arctus, or http://www.uesp.net/wiki/Lore:Talos#Talos / http://www.uesp.net/wiki/Lore:Tiber_Septim. Or is all three of them. These three interact in events centered around the http://www.uesp.net/wiki/Lore:Numidium, first in the year 2E 896 when Talos conquered all Tamriel using it and later during the http://www.uesp.net/wiki/Lore:The_Warp_in_the_West_(event), in which the http://www.uesp.net/wiki/Lore:Mantella#Mantellawas used to power the Numidium using Wulfharth's soul. Later, Wulfharth's soul was released and he dies. Or, it is said that all three of these people became the Ninth Divine, Talos, which brought Lorkhan back into the pantheon as well.
I probably left out a lot, and am wrong in more, so wait for others before you do anything, but this is what I had thought.