Pages from the so-called Devil’s Bible, which is said to contain ‘Satan’s Prayer’, are missing.
The Codex Gigas is the largest medieval manuscript in the world, measuring 9ins thick and 36ins tall.
It is said to contain all human knowledge and was written in just one night during the 13th century, according to legend.
The book was supposedly compiled by just one monk, who offered to write it to avoid the death penalty after breaking his vows.
As midnight approached, he turned to The Devil and sold his soul in exchange for help, the story goes.
Satan agreed and left a chilling picture of himself within the book’s hundreds of pages.
But the book has 10 pages missing from it, which occurred during a fire in the year 1697 when it was one of the few texts saved from their library storage place.
Conspiracy theorists believe the pages were actually deliberately removed, with the fire used as a distraction to do so.
One wrote: “Those who have studied it believe it once contained 320 sheets, 10 of which were removed, that likely contained the monastic rules of the Benedictines.
“Others, however, would argue that the missing pages contained apocalyptic text, known as ‘The Devil’s Prayer’, and were intentionally removed.”
Another said: “It seems strange that the most famous book in medieval times was never copied in more than 450 years from the time it was allegedly written by the Devil.
“Not by the Cistercians, not by the Benedictines, not by the scribes in the court of the Emperor Rudolph II nor by the Swedes.
“For to this day, there is no record of what was written in those twelve missing pages except for rumours that it contains ‘The Devil’s Prayer’.”
However, others are unconvinced by the theory that a mysterious figure took pages from the book.
Another commentator argued: “There’s nothing surprising that some pages are missing in an 800-year-old manuscript.
“Most of them are within the Bible portion, perhaps unsurprisingly as that makes up most of the book.”
The Gigas was initially stored at the Podlažice monastery in what is now the Czech Republic.
It was seized by Sweden’s army in the 17th century and is now being kept at the Swedish Royal Library in Stockholm.
It contains a complete version of the Catholic Church’s official Latin Bible, and a range of texts on the teaching of medicine and magic.
Handwriting analysis suggested a single writer had put together the entire manuscript, according to a National Geographic report.
But researchers say it would have taken 20 years to complete – or five years of non-stop writing.