When last we left things in the second season of House of the Dragon, Daemon Targaryen (Matt Smith) was still ensconced at the crumbling castle of Harrenhal with a collection of soliders from the Riverlands. His nephew Aemond (Ewan Mitchell) was preparing to march out of King's Landing to take Daemon down, both because he's a major threat to Aemond's war effort, and because Aemond has always had a yen to be better than his uncle.
If you've read George R.R. Martin's book Fire & Blood, you know basically how things will go. Aemond will march on Harrenhal, but when he gets there, Daemon will be gone; it's all part of a ruse to lure Aemond away from King's Landing so that his half-sister Rhaenyra Targaryen (Emma D'Arcy) can take the Iron Throne while he and his army are away. Aemond then sets up shop in Harrenhal where he encounters Alys Rivers (Gayle Rankin), the mysterious witch woman Daemon spent time with in season 2.

Without spoiling things too much, Aemond and Alys end up getting close, although the book leaves the details largely up to the imagination. The show will fill them in; according to House the Dragons, which keeps a close eye on all things House of the Dragon, Ewan Mitchell and Gayle Rankin have been filming scenes together on the Harrenhal set.
And there's more! House the Dragons also reports that "[a] large stunt team is filming action scenes involving Riverland soldiers and knights." The Riverlands are at the center of the Seven Kingdoms, which means they're almost always at the center of any war in Westeros. The Dance of the Dragons is no exception. There are lots of battles House the Dragons could be referring to, including the Battle of the Red Fork, the Battle at Acorn Hill, the Battle by the Lakeshore, and the Butcher's Ball. I imagine the show will consolidate some of these battles, since there won't be time or money to show them all, but the point is that things are about to get bloody, especially if you live in the Riberlands.
Cameras have been rolling on the third season of House of the Dragon for a while now, but there's a ways to go. Look for it to premiere on HBO and HBO Max sometime in 2026. Before that, HBO will air the first season of a new Game of Thrones prequel show called A Knight of the Seven Kingdoms earlier next year.
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