(Mild Spoiler Warning)
I declined Brandon's quest in Silverglen and now only my 'main' character can handle Tenebrium weapons. Unfortunately I'm now running into things that seem to *require* them.
I'm so frustrated.
I read it somewhere that if you declined Brandon's quest, not all your character will get the +1 Tenebrium skill.
So if you want all of them to get the skill, you'll have to reload from a previous save, or worst (or for the better depending on your take), restart the game.