Adam B. Vary Senior Entertainment Writer SPOILER ALERT: This story discusses major plot developments in the first six episodes of “House of the Dragon,” currently airing on HBO and streaming on HBO Max. If one of the big narrative challenges of “Game of Thrones” was managing the geographical sprawl of its many interweaving storylines, the challenge for the first season of its prequel series, “House of the Dragon,” is stretching the story of the civil war within the Targaryen family over multiple decades. George R.R. Martin’s bestselling book about the conflagration, “Fire & Blood,” spans generations — children are born and grow into adulthood within the two warring factions of Princess Rhaenyra Targaryen and her former best friend turned stepmother, Queen Alicent Hightower.