Hum, I thaught it was just a usurped realm of oblivion... there certainly is alot of that in Elderscrolls lore.
(Aedra stole it from Lorkhan, and deside to call it Mundus or it was always called Mundus... anyways Tamriel. not part of the deadra who include the princes and Mehrunes dagon who tried to usurp it from the Aedra and Akatosh, who is the leader of the nine divines and the blood relative of talos and the dragonborn and dragons i guess as well as the barrier keeping the daedra and deadric princes from coming in and wreaking everything, the heart of Lorkhan, a very powerful and indestructable gem shaped stragely like a big heart, is the physical remains of Lorkhan the Spirit of Nirn the earth bones god of all mortals, who was probably at one time leader or something of the Deadra who probably usurped the plains of oblivion into being, this is all supported by Mankar Camoran most likely told to him by Mehrunes dagon who is not the prince of lies and deceit and evil just ambition and change and the story of Arkay that states roughly that all the divines havent always been the nine divines but were at least one time mortals, and also that when 4 members of the race of elves once known as the chimer tapped into the power of the heart of Lorkhan Turned their entire race, who are no way connected through telepathic means like the dwemer were, into dark elves, it is writen by vivic that Azura told them not to and it was a cure for their tapping into the heart or by her or something, strange there is another race that has dark skin and glowing red eyes... who was that oh yes the daedra.
Hell there is even a story that says Ariel tried to destroy the heart of Lorkhan and it laughed and said it was the umbilical to the world. )
Anyways this is no longer an argument about it.