If you could put points in Tenebrium - before your characters would actually discover that part of the story and find out there is such a thing in the game world - it would be the worst possible metagaming feature ever made.
- i would agree that it isnt right that weapon skills dont affect tenebrium weapons but i guess they couldnt figure out an easy way out of that one.
There is a very simple solution to both of these problems.
Don't make a Tenebrium skill. It's not needed. It adds nothing to the game, and REMOVES quite a bit. Instead, simply give each character a talent : Tenebrium Training - no longer recieve Rot from Tenebrium.
You could add T to weapons, but it works exactly like every other method of enchanting.
When I add fire damage to my sword, I don't have to rely on 'Fire Skill', it just adds bonus fire damage. That's how Tenebrium should work.