House of the Dragon: TV Show vs. Book Differences
8 Ways "House of the Dragon" Puts a Spin on Plot Points From the Book
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This particular storyline is more a case of "House of the Dragon" making explicit what "Fire & Blood" merely implies. In the book, Rhaenyra's sons Jacerys, Lucerys, and Joffrey are heavily rumoured to be the illegitimate offspring of her ongoing affair with Ser Harwin Strong, particularly since they are dark haired and do not have Targaryen colouring. However, since Laenor's mother, Rhaenys Velaryon (previously Targaryen), has dark hair due to having a Baratheon mother herself, the possibility remains that the boys could be Laenor's.
"House of the Dragon" pretty much confirms that Rhaenyra's three Velaryon sons are actually Harwin's. He is clearly very close with the boys, and since Rhaenys is a Targaryen blonde in this adaptation, there's no other plausible source for the boys' dark hair.