The Codex Gigas is the largest medieval illuminated manuscript in the world. It weighs about 165 lbs.
It was written in the 13th century but the author is disputed. Legend has it that a monk (possibly Herman the Recluse) was sentenced to death for breaking his vows. He was to be walled off alive. While waiting for his sentence to be delivered, he knew he couldn't finish his book, so instead of praying to God he prayed to the devil. The devil appeared to him and the monk sold his soul in exchange for being able to complete his book in a single night.
The book is known for its creepy caricature of the devil.
Despite sometimes being called "The Devil's Bible", it doesn't worship the devil. It has the complete Vulgate Bible and many other popular medieval works at the time. It also tells you how to protect yourself against the devil, such as exorcisms and how to repent for your sins. Some scholars believe that the image of the devil is there to scare you, as if to say this is what you will come across if you sin.
The infographics video on it: