Whoa, thanks for all the replies! After having two saves that can now use Tenebrium (but can't find true Tenebrium weapons yet, haha), I can fiddle around with it (and have) by adding it to other weapons, etc.

Another question, though. If I add a Tenebrium bar to a weapon, it gains extra Tenebrium damage ("Shadow" damage, according to the combat log) on top of the physical damage (which the Tenebrium skill boosts).

Do true Tenebrium weapons (that have the dark models, can't be upgraded, etc.) deal 100% "Shadow" damage, or is it still considered physical (be it crushing/slashing/piercing)? (ie: would the combat log still say "X hit monster for 150 physical" or whatever.)