And we still don’t know who by or where the Ulfberht swords were made. The steel was centuries ahead of its time.
Ulfberht steel was crucible steel. It was remarkably fine grained and had very few inpuruties. Likely imported from south Asia. Europe did not work out crucible steel for centuries after the Ulfberht swords were made. You can learn a great deal in the NOVA special on PBS.
https://www.dailymotion.com/video/x6ciave
Also MAN AT ATMS REFORGED did a short on making an Ulfberht sword from scratch. They made their own steel from ore and all.
The flexibility and strength in sword steels today we rather take for granted. Ulfberht steel would have seemed to be magic to the smiths if the age. An Ulfberht sword would be equal in value to a small castle back then. Of course, where there is quality there were counterfeits. Humans haven’t changed much in the last thousand years or so that way. The NOVA special references fakes. Just like fake Rolex qatches today, there were fake Ulfberht swords.