Death mark, is a very annoying skill to use.
It was made to be an assassin skill I would assume as it falls under the assassins tree in game.
I have some suggestions to make it feel better
Apply the mark on Melee hit.
reapply the mark on ever hit (it give enemy a de-buff) have the de-buff last 6 sec.
have the de-buff count as a wound "weakspot"
remove the "if un-toggled the mark is removed"
Change it so the mana cost is not insane ~ 100 mana when toggled.
Few other notes~
Damage buff does not apply to all damage types currently. one to note is pure damage.