I'm not sure how well-known this tidbit is, but--in case it's not--I wanted y'all to know that mark of pain works off of barbed trap. For some reason, it does -not- activate off of spiked trap, despite it being physical damage as well.
In any case, for all those pro-trapper groups, especially those that run in UW or the new Tombs/Black Hole, mark of pain -does- fire on barbed trap. Obviously, you have take into account distance from foes and whatever, but it's awfully nice to instantly double damage from EVERY barbed trap that was set, and apply that to every monster in the radius. Even better, since it's shadow damage, there's no armor reduction..

Pwnz.
Finally, a new use for the spell. :d