A Cautionary Tale
When witches get together, they naturally talk shop, just like everyone else, and this, after all, was a gathering of all the world's witches.
There in the big cave where they meet, it became a contest: whose magic could do the most harm? Spells, poisons, incantations: one after another, the magics just got worse and worse.
(Terry Pratchett calls such gatherings “witch trials”, where—like everyone else—witches vaunt and strive to outdo one another. The food is always good, at least.)
Finally the last witch stood up. Her hands were empty. She had no cauldron, no wand, no pouch of baneful herbs.
“I have a story,” she said.
The witch told her story. It was horrible: a story of death and rapine, suffering and cruelty. All the witches agreed that this was by far the winning wickedness, the worst magic yet.
There followed a pause and a stirring among the gathered multitude.
“You've gone too far,” the witches told her. “Even for bale-workers like us, this is beyond the pale. This story is just plain too evil. You have to take it back again.”