(NB this is exactly what I wanted to do, so kudos to you!)
I am currently playing some Morrowind so I'd take information from there (races, artefacts, Gods)
I'd start with the Gods if I were to do it
Gods are like people, they play, they teach, they learn and they fight. Very long ago a God named Alvarec had a quarrel with his fellow Gods. They beat the consciousness out of him and decided to imprison him for eternity in a place where nothing exists. And so they did. Alvarec was thrown into nothingness an nothingness engulfed him. Millions of years passed with him not being able to think, feel or breathe. Slowly he re-discovered thought and with the thoughts came action: he turned nothingness into something and from this moment the world of Alva was born. There was nothing on it, it was bare and made of "universal substance", but it was something. With the birth of Alva Alvarec could think further and act further. He learned how to travel in space, his limbs and organs began to function again, he regained memories of his previous existence. As a sign gratitude to Alva he cut a piece of his heart and inserted it into the middle of the world heating up Alva with his blood. Thus for as long as Alva was alive he was alive and for as long Alvarec was alive Alva would live. He remembered the spirits and other Gods and created the spiritual sphere - vacuum where his spirit could walk unnoticed to common eyes. He remembered the feelings that he felt and created physical world where he could live in open and regain his energy. He looked at Alva and thought "my dearest, I owe my life to you" and from those thoughts Love entered Alva and produced his first offspring - Alvoids: Gods and Godesses of Alva. Alvarec spend some time with his children teaching them his skills, giving them his sacred knowledge and walking with them through corridors of Space and Time to other worlds.
Millions of years passed, many deeds were accomplished then and first creatures were born or entered the Spiritual sphere of Alva. Once Alvarec said to this children: "You are now mature. I have to leave you. I have unfinished business to take care of. I trained you well. Take care of your mother, love her. Love her and I shall come backto you alive" And so Alvarec left Alva and his spouse and children cried so much that their tears fell upon Earth and created Oceans and Seas. But they believe and know that their father would be back. For millenia children of Alvarec did little but take care of their world.
At one time the eldest son Arrus gathered his brothers and sisters and minor spirits and said: My father nourished us, took care of us. Through us he revealed to me he became more nobler and better than he was. He also said that there will be others after us, after our time has passed and that we should prepare the world for those who are to follow so that they live and multiply. He said they will live in the physical world that is not bleak and barren. Some upon leaving the physical plain would move on to higher plains outside of Alva to pursue a noble but unknown destiny. He said that we are to become teachers and shapers. What say you, will you take on thus burden?"
The Gods and Spirits danced in anticipation of more living creatures (at that time "living" meant existing in Alva, not in the physical plain". And so they took to work.
Here's where the story ends, you are welcome to include second gereration Gods, newcomer Gods, stories about how trees, elves, humans and lionmen were created.
What do you think?