In House of the Dragon, Vermithor is an old dragon that once served King Jaehaerys I Targaryen. By the time of the Targaryen war of succession, Vermithor is one of the oldest mythical beasts in Westeros.
Vermithor first appeared in season 1 of House of the Dragon, where Daemon Targaryen commanded the beast's attention by singing "Hāros Bartossi." Vermithor was initially hostile, even breathing fire at Daemon, before quieting down and meeting his steely gaze.
In George R.R. Martin's Fire and Blood, Vermithor met his end at the Second Battle of Tumbleton, one of the last and bloodiest battles in the Dance of the Dragons. Going by the point at which this battle occurs in Fire and Blood, Vermithor's death will likely occur very late in House of the Dragon's run, perhaps even in the final season.
How might Vermithor die in House of the Dragon?
Vermithor, along with three other dragons, perished in the Second Battle of Tumbleton, as depicted in Fire and Blood. This battle, occurring in 130 AC, was between the greens, who fought for Aegon II Targaryen, and the blacks, who fought for Rhaenyra Targaryen.
By the time of this battle, Vermithor's rider was Hugh Hammer, the bastard of a blacksmith. While Hugh initially fought for Rhaenyra Targaryen and the blacks, he later defected to the greens and betrayed her.
Shortly before the Second Battle of Tumbleton, Hugh took advantage of Prince Aemond's death to seek the crown for himself. This made him an enemy of both Seasmoke, the dragon ridden by Ser Addam Velaryon of the blacks, and Tessarion, ridden by Prince Daeron of the greens.
Addam Velaryon led an assault on Tumbleton's Hightower garrison, where Hugh Hammer was stationed. Hugh was unceremoniously dispatched by Jon Roxton of the Caltrops when he tried to ride Vermithor and repel the assault.
Addam's knights tried to kill Vermithor too, but their attack failed, enraging the dragon. In his fury, Vermithor indiscriminately slaughtered everyone in sight, including Lords Piper and Deddings. As Vermithor took flight, Seasmoke attacked him, but Seasmoke was no match for Vermithor's strength.
When Tessarion entered the fray, all three dragons fought to the death. While Vermithor succeeded in finishing off Seasmoke, his wings were utterly tattered in the fighting. Vermithor's efforts to flee the scene, taking Seasmoke's head as a trophy failed, leading Vermithor to collapse and die.
Silverwing survived the Second Battle of Tumbleton. According to books, she landed by Vermithor and tried to lift his wings three times with her nose, as if begging him to rise and fly again. After Tumbleton was rebuilt following the battle, the site of Vermithor and Seasmoke's death became a popular tourist attraction.
It is unclear how House of the Dragon will adapt the Second Battle of Tumbleton, given how late it occurs in the Dance of the Dragons.
While the HBO Max series has taken creative liberties with George R.R. Martin's Fire and Blood in the past, House of the Dragon has been largely faithful to its source material, strongly indicating that viewers will witness Vermithor's demise near the series's end.