Sweden's National Library protects the Codex Gigas—165 pounds of vellum featuring the complete Bible and a terrifying portrait of Satan. Scholars estimate the work should've taken thirty years. Herman the Recluse finished it in one. 

The traditional story involves a desperate bargain with darkness. The real story connects this manuscript to a limestone crack in Bohemia where creatures emerged nightly, a duke who sent prisoners into the depths, and a fortress built to seal Hell's gateway. 

When the Nazis occupied the site in 1940, they brought excavation equipment and Himmler's personal occult division. What they found made them destroy everything and flee. 

https://www.youtube.com/watch?v=tgXZ6bVuuN8