According to one version of a legend that was already recorded in the Middle Ages, the scribe was a monk who broke his monastic vows and was sentenced to be walled up alive. In order to forbear this harsh penalty he promised to create in one day a book to glorify the monastery forever, including all human knowledge. Near midnight, he became sure that he could not complete this task alone, so he made a special prayer, not addressed to God but to the fallen angel Lucifer, asking him to help him finish the book in exchange for his soul. The devil completed the manuscript and the monk added the devil's picture out of gratitude for his aid. In tests to recreate the work, it is estimated that reproducing only the calligraphy, without the illustrations or embellishments, would have taken 5 years of non-stop writing.
In popular fiction, the twelve missing pages of the Codex Gigas are rumoured to contain an apocalyptic text called "The Devil's Prayer"
The script writers from Penny Dreadful must have been reading up about the Devil's Bible on Wikipedia.
The introduction of this storyline was good for the pacing in season 2,
until they ruined it and made it all about the Wolf of God.