Unfathomable Darkness' proc damage is being increased by Hurricane. When you proc the set with Hurricane the dmg will ramp up with Hurricane's damage ramp up, causing the set to do far too much damage than intended, this is especially damaging in PvP where this set can hit for up to 7k per tick.
When the set is proc'd with light attack (or virtually any other skill other than Hurricane):
When the set is proc'd with Hurricane:
This was performed on a dummy however the same effect also occurs on players, overland mobs etc.