I suspect this is a bug. If I had to hazard a guess Tenebrium was added late game as a means to combat the quadratic wizards/linear warriors issues we always see in these types of games by providing those that do physical damage a means to provide overwhelming damage compared to wizards.
Look at it this way. A knight with +5 2h and +5T would be doing effectively triple damage every single hit, and six times weapon damage (or more!) on a critical. That's a tremendous amount of damage.
Tenebrium guerrilla backstabbing would be insane.
