Good novel but I disagree with the fact this isn't a fan fiction, much of the lineage here is only from Vikings and in reality all of Ragnar's sons that are notable (aside from a passing mention) come from his 2nd and third wife (Ragnar met lagertha when he went to avenge his grandfather and she was key to his victory)
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LIKERagnar Lothbrok was a real person. That already eliminates the term, fanfiction because he's a historical figure that tales were told about in various forms in terms of media representation. I did get my inspirations from Vikings, The Last Kingdom, Barbarians (Netflix) amongst many others and then doubled down with my own research. If you read further, all this is explained during the war arc. And I included a lot more accurately historical figure, that were alive during the life of Ragnar. So this is history told in my "own" way. Let me give an analogy. A famous dogs dies and numerous stories are told about that dog. Regardless of similarities, this is inevitable as each stories would share similarities to accurately portray how the dog was like be it in their own way of fiction or otherwise. Or someone rewriting how Thor or Odin was and someone says it's a fanfiction of Marvel. God of War for example I do understand what you're saying, but Ragnar Lothbrok is a historical figure. There's no "copyright" to his name or adventures.