Would this be part of base damage, like a slayer arrow, or be a proc effect, and therefore not multiplied by the inevitable critical hit that would come with a vorpal?
I don't really like it as a proc effect, as above poster mentioned it would essentially be like a lightning strike. Meh. Rather it was added to base damage, but probably toned down some, like maybe barb level x 10.
Yes, as written it would greatly benefit single weapon and two weapon fighting above two handed.