At first I assumed it would be like, ALL of it, but now I'm not so sure. After all, I have exactly zero shadow resistance at the moment (I really should work on that, shouldn't I?) and I die to two hits from a nightblade, which is only even HALF shadow damage and half Normal (which I've got plenty of resistance to) but if I'm playing recon, and I put the debuff on someone wearing normal/piercing resist armor like wolver or jelly sets, it seems to take me about... three pokes in the face with my Rigadoon. Also, I tried using my Kamarin as Recon (Which I would advise you to be REALLY CAREFUL about. Having lowered attack speed while using anything from the troika series will leave you very vulnerable to counter-attacks) and it still took at least two hits to kill people. If they had been left with zero normal resistance, I don't think they'd survive the first hit of something that deals so much sheer damage per hit.
So, maybe the debuff HALVES the resistance? Or maybe it lowers it by a fixed amount, that is such it would completely remove low levels of resistance but if they've got a lot of one kind they'd still be left with some of it? What do you guys think? Do your personal experiences support or contradict my findings?
The Recon death mark removes all defense. Once you have the death mark on you, the only thing keeping you alive is your hit points.