1. The Prior of Sion, a centuries-spanning secret mystical order of knights claiming descent from the secret son of Christ, is definitely a hoax. Wikipedia has a pretty darn good article on it, the jist of which is that there's no evidence of its existence prior to 1956. I like to think that Pierre Plantard, the guy what did it, would have made a great DM if he had been born a generation or two later. Anybody who tries to sell you on the Priory as something besides the second greatest hoax of the the twentieth century is either a tool, a salesman, or Dan Brown (i.e. both). The greatest hoax of the 20th century is the Protocols of the Elders of Zion. And by "greatest" I mean "worst". Published at the beginning of the century, it purports to be the meeting minutes of the international Jewish conspiracy to destroy/enslave all us Gentiles. You ever meet anyone who gives the Protocols the least bit of credence and you have my permission to punch them in the face. It's what Indiana Jones and Captain America would do.
2. I never meant to imply yesterday that I suspected Tolkien was the creator of the Voynich Manuscript. My theory was that he encountered the Manuscript as an academic curiosity and contacted the unnamed military man in the quoted anecdote to see if the text was a cipher of some sort. Though someone wanting to make a little money could probably knock off a "non"-fiction book arguing that Tolkien faked all the pre-20th century history of the codex.
3. Like James in yesterday's comments, I don't buy the theory that explains the Voynich Manuscript as some charlatan's attempt to gin up a fake grimoire. That would violate the first principle of faking: make sure your fake resembles the real deal. When someone shows me an earlier grimoire that resembles Voynich, then we have something to work with. Until then, it would be like me taking a transcript from the Gong Show and trying to pass it off as a lost work of Shakespeare. It's just not a good match for serious fakery.
4. Now let me tell you my pet theory. The mystery of the identity of Jack the Ripper and the mystery of the authorship of the Voynich Manuscript are two of a kind to me. Until I see better evidence otherwise the answer to both is this is the work of an anonymous crazy man. I double dog dare the universe to prove me wrong and come up with something awesomer than that, but until I see good solid evidence I feel that Occam's Razor leans towards the side of the most boring explanation. Though I don't find that answer as boring as most people. The Whitechapel Murders are horrible tragedy, but under my theory the Voynich Manuscript is really a triumph of outsider art, like if the HYBRID-RPG was written in its own script and fully illustrated.
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