The Game of Thrones prequel TV series House of the Dragon is based on George R.R. Martin's book Fire & Blood, so while audiences who have already read that novel have a good idea of where this first season of the spinoff will go, there's still plenty of uncharted territory to explore. One of those mysteries is about the castle Harrenhal, and while Fire & Blood details that the castle had suffered a brutal fire, it leaves the cause of the inferno up for question. During this week's episode of House of the Dragon, however, we know now exactly what happened on that fateful night.
WARNING: Spoilers below for House of the Dragon Episode 6
Harrenhal, which was once burned down by dragons in Aegon's Conquest, has long been considered a cursed castle. Every devastating event that ever occurred at the castle was blamed on the curse, including a fire that killed Lyonel and Harwin Strong. A reason for the fire was never officially given in the books, but fans had their theories. The new episode of House of the Dragon confirmed one of those theories was actually correct. Larys Strong, son to Lyonel and brother of Harwin, was behind the fire.
It's all-but revealed, in the book and on House of the Dragon, that all of Rhaenyra's children were fathered by Harwin Strong, not by husband Laenor Velaryon. While Viserys won't address the issue head-on, Alicent wants to do something about it, and she informs Ser Larys Strong of the problem. He decides to take matters into his own hands.
Late in the episode, Larys approaches a group of prisoners that are set to be executed. He tells them of a proposition that will give them their freedom, in exchange for doing him a favor and having their tongues cut out. Those same men appear at Harrenhal in the episode's final moments, starting the fire that kills Harwin and Lyonel.
This solves multiple problems at once for Alicent, even though she makes it clear that she didn't ask for the fire to happen. This takes Harwen out of the equation, keeping Rhaenyra from being happy while also keeping the family clear of scandal. This also allows someone else to step in as Hand of the King, the position held by Lyonel that he tried to give up before his death. Viserys would not accept his resignation. Alicent can now influence the appointment of someone else to the position.
Larys is now the only remaining man in House Strong, given that Harwin's sons aren't considered his own. He will inherit Harrenhal and everything else that belonged to his father.