I took the time to write the new background story. Here it is:
(short version)
It all began with Itarkra Tarm, a dwemeric scientist and a mighty magician, who searched for the secret of eternal life. He was born in a Dwemer settlement near Red Mountain 50 years before the Siege of Red Mountain. At the time when the situation in Vvardenfell heated up, he flew to Cyrodiil. He didn't want to become a part of the machinery of war and to refrain from his comrades who didn't accept his works and rather made fun of him. In the seclusion of the mountains of Cyrodiil at the border to Morrowind he built his hideaway which consisted of a laboratory and a living area. After the construction was finished, he used his magical power to weave one part of his hideaway with the structure of the Outer Realms. If required he could change this area into a plane of the Outer Realms completely which was maintained by the magic of Welkynd and Varla stones because it was an enormous energy sapping process. This special area was his experimental space being untouched by outside influences which could disturb his experiments. His staying in this plane prevented that he disappeared like the other Dwemer. He did know that Yagrum Bagarn didn't disappear as well but he avoided any contact with him.
The years had passed. Itarkra's research didn't make any progress. One experiment after another failed. His test subjects, mostly beggars, became rarer and rarer. He didn't even shy away from abusing his assistants for his experiments who were simple humans. All died like flies or - when they didn't die - became will-less zombies. One of his assistants couldn't stand by and watch any longer. He flew to Mournhold one night in order to report to Almalexia what Itarkra had done. He hoped that she granted him asylum.
Almalexia was very upset. In her eyes only gods should be immortal. She sent her Hands to stop Itarkra's efforts and to kill him. The flighty assistant beaconed the garrison to Itarkra's hideout in the mountains. Itarkra awaited them already. He had somehow found out what Almalexia had planned. Itarkra stood at the entrance to his laboratory with his head held high. But something had occured to him.
In the present. You receive an ominous letter. The letter writer asks you to come to Weynon Priory because he wants to discuss an important matter with you privately because he doesn't trust the courier. In Weynon Priory you meet an odd-looking figure sitting at the table smoking a pipe. A hood is hiding his face. He introduces himself as your brother Xantir and is acting on Itarkra's behalf who is according to Xantir your father. Curious to find out more you start your way to the sanctuary in the mountains accompanied by Xantir.
The long version of the story has 5 pages in Word. Maybe I split it into different parts and hide the parts as books at some Abyss Demon locations in game.