Spoiler My first thoughts when I saw "Listen" wrote on the Doctor's chalkboard were this, "Listen to the silence." I think it was a little bit of sourpuss carrying over, not wanting to let go of Eleven's reign as Doctor, but it was an eery thought. Perhaps these creatures who were made to hide are going to be Twelve's 'Silence' or maybe they were just one-episode props ... though from past experience, that is highly doubtful. I don't believe that it was one of Pink's friends pulling a prank because it had the Doctor truly absolutely terrified, no matter how much he didn't want to show it. Then again, children do tend to terrify the Doctor. I also have a strong doubt that the Listen are tied into Missy but I do have a strong feeling that they will play a big role later on down the road. Look at how the Silence was used as a long running theme. The first time the silence was mentioned was in "The Big Bang" (2010) and would be a recurring theme until "The Time of the Doctor" (2013). It is highly possible we'll be hearing more from the Listen in the near future. Moffat is always being Moffat.