In the books, during her reportedly unfulfilling marriage to Laenor, Rhaenyra is rumored to have taken Ser Harwin Strong as a long-term lover. Given that she's married, their relationship is certainly forbidden, but it reportedly lasts for several years. Rumors also spread that all three of Rhaenyra's sons, legally the products of her marriage with Laenor, are actually Harwin's children.
Like so many things in Rhaenyra's life, her relationship with Harwin also does not end happily, at least in the book. After a fight sparked by more accusations that Harwin fathered Rhaenyra's sons, he is removed as her sworn shield. He dies later that year, along with his father, in a terrible fire at Harrenhal. The fire is reported to be an accident and blamed on a curse, but some suspect more human (and vengeful) causes.
In "House of the Dragon," we learn the rumors of Harwin and Rhaenyra's relationship were true. Harwin and his father are killed by Harwin's brother Larys, Alicent's strongest ally. The curse of Harrenhal, their House seat, lets him get away with it.
