Wyrd Sisters: (Discworld Novel 6)

Things like crowns had a troublesome effect on clever folks; it was best to leave all the reigning to the kind of people whose eyebrows met in the middle. Three witches gathered on a lonely heath. A king cruelly murdered, his throne usurped by his ambitious cousin. A child heir and the crown of the kingdom, both missing... Witches don't have these kind of dynastic problems themselves - in fact, they don't have leaders. Granny Weatherwax was the most highly-regarded of the leaders they didn't have. But even she found that meddling in royal politics was a lot more complicated than certain playwrights would have you believe, particularly when the blood on your hands just won't wash off and you're facing a future with knives in it...