In the fictional Discworld series by Terry Pratchett, the Octavo is the Creator's own grimoire and thus the most powerful book of magic on the Discworld. Despite its importance, its appearance is rather plain; it is a large, yet unimpressive book bound by brown leather, with an illustration of Bel-Shamharoth on the cover.

It was reputedly left behind, with characteristic absentmindedness, shortly after the Creator completed his major work. The Eight Great Spells that initially made the Discworld are imprisoned on its pages, giving the book sentience.

It somehow came into the possesion of Unseen University, where it was stored in a little room off and under the University's Library. Given the nature of the book, the room is full of precautions; not so much for the protection of the book as much as for the protection of its visitors. The walls are covered with protective symbols and lead pentagrams (a reference to precautions taken to shield nuclear reactors; Pratchett once worked for a power generation company). On the floor is the Eightfold Seal of Stasis. Even with all these measures, no one may stay in the room for more than 4 minutes and 32 seconds, a figure deducted after two centuries of cautious experimentation.

The Octavo is attached (perhaps a more accurate term would be "tethered") to a lectern in the shape of an unpleasant winged creature by a very heavy chain fastened by eight padlocks (one for each Head of each Order of Magic). It is also held shut by metal clasps.

While on his first year at the University, Rincewind the wizard tried to open the Octavo for a bet; he miraculously succeeded to bypass all safety measures and open the book, whereupon one of the Eight Great Spells leapt from the book and lodged itself into his mind. No wizard could coax it out. For this, Rincewind was expelled from the University and was unable to learn any other spells because they were afraid to share his head with one of the Eight Spells. Eventually, the Spell returned to the Octavo and Rincewind said all of the Spells to prevent Discworld's imminent destruction (as described in The Light Fantastic). The book was subsequently swallowed by Rincewind's Luggage, but it was spat out a few days later. Presumably the Octavo has returned to its chamber at the University.