My Parents are DEAD! is a running gag that originated in Video Games Awesome's run of Batman: Arkham Asylum, alongside the Nananananana Batman running gag. However, unlike that gag, this one persisted into the run of Batman: Arkham City. It is usually exclaimed by one of the members of the cast in a loud voice whenever Batman's parents is mentioned, or he acts particularly violent.
This meme has also leaked into a few other unrelated playthroughs.