The Codex Gigas  is the largest extant medieval manuscript in the world. It is thought to have been created in the early 13th century in the Benedictine monastery of Podlažice in Bohemia (modern Czech Republic). During the Thirty Years' War in 1648, the entire collection was stolen by the Swedish army as plunder and now it is preserved at the National Library of Sweden in Stockholm. It is also known as the Devil's Bible because of a large illustration of the devil on the inside and the legend surrounding its creation.

The codex is contained in a wooden folder, covered with leather and ornamented with metal. At 92 cm (36.2in.) tall, 50 cm (19.7in.) wide and 22 cm (8.6in.) thick it is the largest known medieval manuscript. It initially contained 320 vellum sheets, though eight of these were subsequently removed. It is unknown who removed the pages or for what purpose but it seems likely that they contained the monastic rules of the Benedictines. The codex weighs nearly 75 kg (165 lbs.) . The Codex Gigas has 310 parchment leaves, perhaps made from calfskin.

The codex was believed to be created by Herman the Recluse in the Benedictine monastery of Podlažice near Chrudim, which was destroyed during the 15th century.

The Codex includes the entire Latin Vulgate version of the Bible, except for the books of Acts and Revelation, which are from a pre-Vulgate version. The entire document is written in Latin.
The manuscript includes illuminations in red, blue, yellow, green and gold. Capital letters are elaborately illuminated, frequently across the entire page. The codex has a unified look as the nature of the writing is unchanged throughout, showing no signs of age, disease or mood on the part of the scribe. This may have led to the belief that the whole book was written in a very short time. But scientists are starting to believe and research the theory that it took over 20 years to complete. Page 577, otherwise empty, includes a unique picture of the devil, about 50 cm tall. Several pages before this are written on a blackening vellum and have a very gloomy character, somewhat different from the rest of the codex. The reason for the different coloring is that when vellum is exposed to light it "tans", as vellum is made from animal skins, so over the centuries the pages that were exposed will have a darker color to them.

According to legend 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 single night 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 archangel 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.

It is also speculated that the inscription of various exorcism spells is the monk trying to exorcise the devil out by himself. (Similar to monks who sinned, and were punished by having them write out the Bible by hand, as it was believed that God's goodness would be transferred to them by doing that.)

Despite this legend, the codex was not forbidden by the Inquisition, and was studied by many scholars.

Modern studies in linguistics suggests the monk was in total solitary confinement, possibly self imposed. In tests to recreate the work, it is estimated to have taken 20 or more years to have written the work.