Vermithor’s book fate is that he ends up dying in the Dance of the Dragons, during the Second Battle of Tumbleton. After Hugh Hammer claims him, Vermithor fights for the Blacks, kills Seasmoke in the battle, and is then brought down after his wings are badly burned and he can no longer stay aloft.

What that means

  • Vermithor is one of the older, larger dragons in the story.
  • He is first associated with King Jaehaerys I, then later with Hugh Hammer.
  • His death is tied to the huge dragon-on-dragon violence near the end of the civil war.

On the show

House of the Dragon has not fully reached that point yet in the published material covered by these sources, so his exact on-screen ending could be adapted differently. The most likely outcome, though, is still that he dies in or around Tumbleton based on the source story.

TL;DR: Vermithor is eventually killed in the Second Battle of Tumbleton after a brutal dragon fight, with Seasmoke taking heavy damage in the same battle.