^Damage is converted back to elemental damage, but amount of extra damage added stays (which is more then essence itself can add normally). And weapon will again use normal skill.
This is useful for character you do not want to invest having tenebrium skill, but do want to give increased elemental damage then usually.
Of course, with recent patch changes, you will probably want to have one character with tenebrium skill, since with new patch tenebrium skill is used for adding tenebrium bars to regular weapons.
