Triwizard Tournament, return of Voldemort (graveyard scene), first major character death, expanded wizarding world.

A short 800-word prequel (2008) exists, but not essential. Fantastic Beasts screenplays are separate canon.

Sirius Black escape, Marauders’ backstory, time-turner, patronus charm – best self-contained plot.

Mystery of the Chamber, Horcrux concept introduced subtly, house-elves, parseltongue.