Grasping Darknesses are fast moving anti-caster warriors. Their skill set causes interruptions, dazed and energy drain which means that a group of them can quickly render spell casters useless if not handled properly. They are known to be servants of the Terrorweb Dryders plaguing the Underworld.
Cast any buffs before engaging them as you might not have the opportunity later.
Grasping Darkness have lower than usual armor against elemental damage (especially compared to the Bladed Aatxes).
They tend to clump, so Area of Effect skills work very well against them.
The ranger skills Dust Trap and Throw Dirt are highly effective in neutralizing their interruptions.
These move 50% faster than normal at all times, making it nigh-impossible to outrun them except with extremely high fire magic (21) and Burning Speed, or by slowing them down.