Your Shadow damage spells have a 100% chance to cause your target to be vulnerable to Shadow damage. This vulnerability increases the Shadow damage dealt to your target by 2% and lasts 15 sec. Stacks up to 5 times.
I haven't tested it, but I'd say it works for raids. Seeing as it is a debuff applied to the mob itself. It's like the rogue ability hemorrhage I reckon; you apply a debuff on the target which increases damage done by anyone done to it.