Seasmoke makes his debut in Episode 3 of “House of the Dragon,” “Second of His Name.” He is ridden into the Battle of the Stepstones by Laenor Velaryon (Theo Nate), son of Corlys Velaryon (Steven Toussaint) and Rhaenys Targaryen (Eve Best). In the books, the striking gray dragon is small, nimble, and bonds with Laenor soon after hatching, around 101 AC. After Laenor dies in 120 AC, Seasmoke retreats to his lair on Dragonstone and refuses to accept another rider for nearly 10 years.
That’s when Addam of Hull — later known as Addam Velaryon — comes along. Addam is Corlys’ bastard son. Like Jon Snow before (or, more accurately, after) him, Addam is determined to prove his worth. Addam and Seasmoke help Rhaenyra take King’s Landing, fighting for her faction — dubbed “the blacks” — throughout the Targaryen civil war. But when Rhaenyra accuses Addam of treason, the legitimized heir of House Velaryon escapes King’s Landing on Seasmoke.
Despite Rhaenyra’s betrayal, Addam remains loyal to her cause. He and Seasmoke ambush Rhaenyra’s enemies at Tumbleton. A rider-less Tessarion, former mount of Prince Daeron, clashes with Seasmoke in the air. When Vermithor, former mount of King Jaehaerys, joins the carnage, all three dragons fight and die.