The Cons:
2. Tenebrium weapons cannot be augmented by Tormented Souls, which grant +1/+1 Stength/Dexterity to the weapon and stack with other augments (+2/+2 from level 17 onwards).
Well, this is not entirely true - i upgraded a crossbow with Bowstring, Tormented Soul and Sextant. And then i added Tenebrium.
Now i got a very powerful weapon for all mobs.
