Book 3: "Sourcery" by Terry Pratchett

[kittiwake]

When the rats flee the Unseen University Rincewind knows that something ominous is afoot. And it's not only the rats that are leaving; the ants, bedbugs, cockroaches and even the Gargoyles are going too, as they know that a sourcerer is coming, the eighth son of an eighth son of an eighth son, a source of magic, and that means big trouble - perhaps even the Apocralypse.