So from what I can read on various internet forums, the only way to be able to use tenebrium weapons is by completing the troll king quest offered by Brandon or use the book found in the immaculate settlement.
I refused the troll king quest and used the book on a wrong character because I did not know what exactly tenebrium was used for at the time.
So basically now its impossible for my fighter character to use tenebrium weapons.
This just seems like extremely poor design.
