I like it; however please consider putting a hard cap on the debuff.
I much perfer -1DC/15sec stacking 10 times. A -10DC would be sufficient in curbing a zerging necro.
Also instead of placing a static quest debuff on all hard/elite/epic hard/epic elite quests, please consider using monster CR. IE. Haunting debuff only applies if you use necromancy instand kill to sniff the life of a much powerful creature. Debuff applies if the creature you kill has CR greater than your character level + 5(that number to be worked out). This will still give necromancers the option to run through level 1 elite quests and slaughter 200 cr 5 kobolds.