Sourcery

'It's vital to remember who you really are . . . it isn't a good idea to rely on other people or things to do it for you, you see. They always get it wrong.'
An eighth son of an eighth son is born, a wizard squared, a source of magic. A sourcerer.
Unseen University, the Discworld's most magical establishment, has finally got its wish: the emergence of a wizard more powerful than ever before. You'd think they would have been a little more careful what they wished for . . .
As the sourcerer takes over the University and sets his sights on the rest of the world, only one wizard manages to escape his influence. Unfortunately for everyone, it's Rincewind.
Once again the cowardly wizard must embark on a quest: to deliver a precious artefact - the very embodiment of magic itself - halfway across the Disc to safety. If he doesn't make it, the death of all wizardry is at hand.
And the end of the world, depending on who you listen to.
'One of our greatest fantasists, and beyond a doubt the funniest' George R.R. Martin
'May well be considered his masterpiece . . . Humour such as his is an endangered species' The Times
Sourcery is the third book in the Wizards series, but you can read the Discworld novels in any order.