All spells that reduce hit points(including DD, DOT, Necromancer Dark Pact
line, etc), do 66% of the damage they would to do a mob, when cast on a
player by another player.
The only exception to this that I know of is the Shadowknight's Harm Touch,
which still does 100%.
Not anymore.
Harmtouch is now 66% damage. It was 80%, but that was deemed too high as
at high levels Shadowknights could essentially "deathtouch" purecasters
with a golem rod/egg pumice, the HT disc, and HT.