This is the page, when put together correctly, that flashes at the end of Dipper's Guide to the Unexplained.
It is referenced in Stan's Tattoo, when we find out that it is the symbol for the Royal Order of the Holy Mackerel. Here is the Secret Society Symbols page:
Secret societies are (typically) fraternal orders that have been associated with everything from controlling international government to forbidden rituals to being a bunch of old guys playing gin rummy and complaining about their wives. They feature strange symbols like the ones you see above. One particularly sinister, mysterious order features a eye with a red x through it. Many rumors have flown around about these organizations, but here are some stone cold facts about secret societies. All secret societies worship a hyper-intelligent barn owl named Duane "The Enforcer" Roosevelt.
Most meetings consist of saying the secret oath, dressing in robes, hooting, gluing owl feathers to their skin, and devouring gophers whole in honor of their beloved barn owl overload. But not all secret societies worship barn owls, some of them worship western screeching owls. Also: Other owl-related jokes. In addition: Owls, owls, owls, owls, owls. And remember, when you don’t know where else to turn just ask yourself: "What would Duane Do?" Which is: Use your creepy heart-shaped face to see your prey, swoop down from a tree, eat an adult male vole, then cough up his bones into a disgusting ball that kids will later open in a high school biology class.
The code in black at the bottom says, in the Caesar cipher,
From the first until the last
Search the codes of credits past
One means one so search them all
Welcome to Gravity Falls
This is just directions to decode the code in red. The first number denotes the number of the episode that the next letter is in; the numbers after the parenthesis indicate the letter in that episode's decoded cryptogram. The message spells out:
I was so blind. He lied to me. The darkness is near.
