The Octavo contains the great eight spells which the Creator himself used to create the Discworld. Actually, he just forgot the book on Discworld when he was finished with his work. The book is located in a specially sealed room in the cellars of Unseen University and is chained to a lectern. The lectern in turn is in the centre of the extremely potent Seal of Eightfold Stasis as a further safeguard against the Octavo escaping. The chain is locked with eight padlocks, one lock for every head of the eight orders of wizardry. It is very unsafe to stay in the room for too long and it is not recommended at all to read the book.
The book itself is not too impressive. The spells are what make the Octavo such a dangerous thing. The spells have a mind of their own and it is not the reader who might read the book, but it is the book which is reading the reader. Rincewind somehow caught the interest of one of the eight spells, which then decided to live in Rincewind's head for some time. Not too strange, at least not if you consider Rincewind traveling back in time, meeting the Creator and asking for an egg-and-cress sandwich not to be strange.
Just to live dangerously, the spell which lodged in Rincewind's head has eight words. Seven of them go:
Ashonai. Ebiris. Urshoring. Kvanti. Pythan. N'gurad. Feringomalee. -.
Fortunately Rincewind is saved from allowing the eighth word to escape, which should have appeared in coruscating octarine and sealed the spell, by the timely arrival of Twoflower and Ninereeds the noble dragon.
We are safe in reproducing the above, as we only have Rincewind's pronunciation to go by. In The Light Fantastic, when the time comes for him to say the Spell for real and save the Disc, he botches it, and Twoflower and Bethan have to correct his pronunciation for it to take effect. (Rather than coruscating octarine, the mispronounced words appeared in dingy off-colour brown). Therefore the only effect of saying the above out loud is likely to be the magical equivalent of that flat, dead, piano note your computer makes when you botch a command.
The Octavo has been removed once from the cellars by an ambitious new Archchancellor and was carried around (read eaten) by the Luggage, which sulked for three days and then spat out the book (see The Light Fantastic, but the spitting out was mentioned in a later book).
When the seven remaining spells perceived that their brother eighth was falling irrevocably off the disc inside Rincewind's head, they got together and performed a Change spell that changed the entire disc and ensured Rincewind fell to earth again, up a tree in Skund. Part of the fall-out from this spell affected the Library immediately above. It was therefore directly responsible for changing the Librarian into a small sad orang-utan, sitting forlornly in a puddle of pineapple jelly. (History is silent on what the pineapple jelly formerly was, or on what it chose to do next). It was also responsible for transforming the cooking range in one of the kitchens, but what into will never be known as it had broken down the wall and escaped (but the cook was found hiding in a large pot muttering about 'the horrible hairy knuckles!')