Being described as 'unique' doesn't necessarily mean it was literally so. It was an awkward weapon, probably used by combat enthusiasts of a sort, who were inclined to learn to fight with awkward weapons, maybe as a form of showmanship. The dart in question is essentially a 1/3rd scale javelin, with a wide head, with barbed hooks running along the lower part of the head. They're blade as well, on the side that'll stab inward, but the alternate side would be blunt, so the hooks would catch, instead of cut, when it was pulled out.

And I agree. Viking Invasion was irritating in so many ways.