Featured at the end of each short from Dipper's Guide to the Unexplained was a brief flash of an image. Combined, they form a page that marks the symbol of one of the most sinister secret societies in Gravity Falls.
Here's "The Lamby Dance" in my language, Gravity Falls will premiere in October in my country but some episodes are leaked in the internet, that's how I got the video. I don't like too much the dub ;_ ;