That anyone can prove that they are descended from King Harald I Halvdansson, called Fairhair, first king of Norway (872-930).
Or indeed from anyone who lived at any time in pre-Christian and early Christian Norway.
If you think that proving such a provenance is even theoretically possible, then you are an idiot and you need to do a lot of reading. And it doesn't matter if you have the paperwork to prove it. If you do, then you really are an absolutely enormous idiot.
(This applies to idiots who are Norwegian or of Norwegian ancestry, of which there is sadly no shortage.)