Le Serpent Rouge

Le Serpent Rouge is a document that gives hints on how to find the treasure of Rennes-le-Chateau. The document contains 13 segments named for the classical signs of the zodiac, plus one.

In Gabriel Knight 3
Grace learns of the document when it is mentioned by Estelle Stiles and Lily Howard at Chateau de Blanchefort. Madeleine Buthane states that the document was deposited at the French national library, but vanished and is incredibly rare. Vittorio Buchelli says the document does not exist.

On Day 2, Grace finds a handwritten English translation on the door to the museum.

The following reveals spoilers to Gabriel Knight 3: Blood of the Sacred, Blood of the Damned

The document is revealed later to have been left by Emilio Baza, the Wandering Jew, for Grace. Though the fingerprint found on the containing envelope belongs to Stiles, this is due to the fact that she picks up the envelope after accidentally running into Baza in Chapter 2. It is unclear whether Baza meant for this to happen, or it was just a coincidence.

Contrasts with the historical document
Le Serpent Rouge: Notes sur Saint Germain Des Pres et Saint Sulpice de Paris was deposited into Le Biblioteque Nationale in 1967 by unknown persons. The document was 5 pages of typewritten French, and was accompanied by several pages of rough maps, a genealogy chart, and other obscure images. The document did not go missing, and is in fact available online from several sources. Although some phrases and even a few sentences resemble the document in Gabriel Knight 3, most of the document has been rewritten. Even the title has been truncated for the game, removing the references to two churches in Paris, Saint Germain and Saint Sulpice, which are supposed to fall on an important meridian.

In the game, Buthane refers to the authors having died under mysterious circumstances. The real document states that the authors are Pierre Feugere, Louis Saint-Maxent, and Gaston de Koker. In Holy Blood, Holy Grail, the authors speculate that these names of suicides were likely taken out of a newspaper and used to enhance the mystery of the document. If this is not the case, no one knows who these men are or why they wrote such a document.

Both the real and fictional documents contain references to Sauniere's church and to Rennes-le-Chateau. These include the church floor's chessboard pattern, the phrase "Par ce signe te le vancrais", the White Fort, Poisson and "his tomb", and the Magdalene statue with the skull at her feet.

While the original document does seem to point to the Priory of Sion ("P to that of the S"), because the Priory is widely regarded as a hoax promulgated by Pierre Plantard it's unclear whether the document has any real meaning.

Textual similarities
The most notable pieces used from the original document by the game are (using translations online)

Aries (real): "In my arduous pilgrimage I tried to clear a path with the sword crossing the inextricable vegetation of the woods, I wanted to reach the residence of the sleeping BEAUTY in whom certain poets saw the QUEEN of a past realm. In desperation of finding my way again the parchments of this Friend were for me, the thread of Ariadne."

Aquarius (game): "In my arduous search, I was trying to hack a way with my sword through the dense vegetation of the woods like some pawn of destiny. I wanted to reach the place of the Sleeping Beauty in which some poets can see the Queen of a lost kingdom."

Taurus (real): "Thanks to him, from now on by measured steps and a sure eye, I can discover the sixty-four dispersed stones of the perfect cube that the brothers of the BEAUTY of the black wood escaping in the pursuit of the usurpers, had scattered on their way whilst they fled from the white Fort."

Gemini (game): "The Mosaic tiles of the sacred place alternate black or white and Jesus, like Asmodeus, observes their alignment. I can find 64 stones of the perfect cube which the Knights of the beauty of the black wood had scattered when they fled from the white fort while they were being pursued by the usurpers."

Gemini (real): "To reassemble the scattered stones, work with the square and compass to put them back in regular order, look for the line of the meridian in going from the south to the north, finally in all directions to obtain the desired solution, stopping in front of the fourteen stones marked with a cross. The circle being the ring [coil of snake] and the crown, and to him the diadem of this QUEEN of Castel."

(game): "Reassemble the scattered stones and, working with square and compass, put them into ranks. The Knightly Order of the Perfect Cube fits within and  gives structure to our Lady's protector, the Heavenly Quartet"

Cancer (real): "The stones of the mosaic paving of the sacred place could be alternatively black and white, and JESUS like ASMODEUS, observing their alignment, my view seems incapable of seeing the summit where the marvellous sleeping one remained hidden. Not being HERCULES with magic power, how to decode the mysterious symbols carved by the observers of the past. In the sanctuary however the stoup, fountain of love of the believers, reminds us of these words: BY THIS SIGN YOU SHALL CONQUER."

Gemini (game): The mosaic tiles of the sacred place alternate black and white and Jesus, like Asmodeus, observes their alignment.

Taurus (game): In the sanctuary in the font, fountain of love, of those who believe reminding us of these words "BY THIS SIGN YOU WILL CONQUER".