Complex sentence: Harry attends the Quidditch World Cup with the Weasleys, but after the game, Death Eaters terrorise the spectators, and the man who appeared in Harry's dream summons the Dark Mark.
Split into simpler sentences: Harry attends the Quidditch World Cup with the Weasleys. After the game, Death Eaters terrorise the spectators, and the man who appeared in Harry's dream summons the Dark Mark.