Eightfold Seal of Stasis
This is a potent magical spell, drawn in the form of a sigil on the floor in the Library cell housing the Octavo. It acts as an extra layer of defence against the Octavo breaking its chains, evading the dread Guardian built into the lectern, and breaking the eight potent locks on a VERY thick and sturdy cell door as it makes a break for freedom. The Seal acts as a magic circle which in this case draws a VERY clear distinction between what stays on the inside, and everything Outside, while serving as unspoken comment on the general inadvisability of the two meeting. It also has the function of alerting senior wizards, if anything untoward happens inside the cell - i.e. it is an alarm bell.
This explains why, when the spells of the Octavo made their escape plans, (by luring in a young and foolhardy student wizard and boring into his brain), only one of the eight spells made it to freedom, before senior wizards intervened.
In occult circles, this sigil is said to have all the stopping power of a well-aimed housebrick. Given Rincewind's propensity for using a halfbrick in a sock as weapon of choice, it can be speculated that this may have created a resonance that drew him into the Octavo's forbidding presence.