Creepy confessors from the Papal Mainframe - The Silence
When first 'seen' in the Impossible Astronaut, these mysterious creatures were a puzzle. They could zap people, they were all over the place on Planet Earth, yet had a strange ability to be immediately forgotten, and were involved with training The Doctor's bespoke assassin - River Song. That last bit we didn't know until much later. Instead we knew that Amy and Rory's daughter was kept by them in a creepy mansion. It wasn't the end of Matt Smiths time we learned what the Papal Mainframe is, and the true role of the Silence.