It's not only the title that tells us Thor: The Dark World will be a more serious story than the first film starring Chris Hemsworth as the hammer-wielding god. Nowadays, it's inevitable that blockbuster sequels -- usually the second installment of a planned series, but, increasingly, the third -- will turn to the dark side, a trend that George Lucas inadvertently started when he guided The Empire Strikes Back to its bleak, unsettling finale. How successful have they been?