It will also debuff any other movement buffs on a mob. Or another player, if they have sow, it will strip your sow. I know that if you have disease for example its so many ticks, and cure and it removes so many ticks. I dont know if its level based or what, I am not sure if it will strip flight of eagles for example... but it strips Sow... Often. I believe the necros in stonebrunt stripped my 39th lvl sow with it. and they are way lower then that.
I am a lvl 37 Rogue. Just got my spine chilled spear, but do not intend to give up my bone dirk yet. The engulfing darkness is extremely usefull when a mob turns to run or you need to turn and run. A good fast high hit weapon in primary and this in off hand make a supurb combination.