Because Gotham can't be normal for once in it's seemingly cursed existence, when kids get drunk and play spooky games at night, they can't just settle for Bloody Mary, and use their own urban legend, "The Midnight Man." So of course... these innocent teenagers end up actually summoning some evil shapeshifting black goo monster. Fun!
Enter Batgirl, who is just sort of hanging out in Gotham, thinking about Dick Grayson and how he's as dead as Damian Wayne... if we're to believe this book (which we shouldn't), and then thinking about Dick Grayson some more. Just keeps thinking about that Dick... anywaaaays, Babs EMF detector goes off, and if you don't know what an EMF detector is... what an episode or two of Ghost Adventures and try not to laugh. It's a big spike on the EMF, especially for Gotham, so naturally Babs has to investigate.
Things go as you'd expect. Batgirl enters house, everyone wakes up, scary monster, run away. It all ends up with Babs and the kids sneaking through the large vents into the kitchen to get some sage, which apparently works well against the paranormal. Sage acquired, set on fire, sends Midnight Man back to wherever. Babs scolds kids for summoning ancient evil, leaves, still thinking about Dick Grayson... Robins... something like that.
OH NO! Midnight Man is somehow in some paint that Gotham works crews are using for street lines and it's forming his ritualistic doorway!