My guess is the Pain Skin has the same effect as Putrid Skin did one the groupable T2 TSS BP. It adds a def proc that taps the mobs attacking you for 400 hps. Assuming it is just an upgrade, it will not stack with Umbral Skin it will overwrite it, Putrid Skin from the TSS BP overwrites Umbral Skin so new one should as well too.
Darkreaper Swifft Darkness of the Seventh Hammer