Don't Forget About Us: Season 9, Episode 19
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At the reveal, Billy listens to audio footage of a woman humming, the door closing in the basement, and a young female voice that appears to be in pain. That bothers Billy, but the audio of a male voice saying "don't forget about us" makes him soften a bit. It would logically be the soldiers, so now Billy knows what he needs to do for these spirits.