Perhaps the Tenebrium requirement on the weapon should subtract from the normal weapon skill (representing the difficulty in handling a Tenebrium weapon safely), and each point in the Tenbrium skill offsets that penalty.
i.e. a tenebrium requirement of 3 on a two-handed sword applies a -3 to the users two-handed weapon skill, but can be lowered to a penalty of -1 to the two-handed weapon skill if the user had a tenebrium skill of 2.