^^^
Hmmm. Still a toss up in my mind. It is better IF there is a monk in the party, but way worse if there isn't because of it only decreasing damage to you and not raidwide.
I get what you are going for though, but perhaps a "does X% less magic damage" debuff would work better as it would stack with DK and affect everyone.
Slightly tangential, but I really feel that DRK could use a increases damage taken debuff to bring more raid utility. I would like to see Delerium have a Dark Arts effect that gives the debuff of "takes 10%(or higher/lower depending on balance) more magical damage" to the target.
