One of the Renfields had scouted ahead and found a path out from the tunnels. "This way! There's daylight, thank the Gods!" The rest of the Renfields ran ahead, glad to be out of this basement. They started to follow them up, Harry was hot on their heels. As they made it out, he realized something seemed wrong. This is a bloody trap, isn't it? As the Renfields scampered away, each of them went a different direction and bled themselves to complete a circle, locking Harry and the rest in just like Severus and Narcissa did to the Inferi at the Awards Banquet. They all had vampire blood in them. The same blood.