Bad Lullaby is an achievement that can be unlocked in Among the Sleep.
How To Unlock[]
This is unlocked during Chapter 4: Into The Closet.
After pulling a handle that creates a bridge to cross the gap, proceed across it. At the other end to your right you will see a picture frame leading into a tilted over clock. Climb up the clock and into the hallway and to your immediate right is a door. Open the door and you will see a music box mysteriously illuminated by a single flickering light. Walk up to the music box and press to play it. As the music plays, the achievement will unlock.