Final answer:
The phrase "Dark things happen in dark alleys" is an example of foreshadowing, suggesting that the superhero's nemesis may be keeping a hostage in a dark alley.
Step-by-step explanation:
The phrase "Dark things happen in dark alleys" is an example of foreshadowing. Foreshadowing is when the author gives hints about the plot developments to come before they happen. In this case, the phrase suggests that something bad or dangerous may happen in a dark alley, which indicates that the superhero's nemesis may be keeping a hostage there.