If you've ever heard the old expression "hell hath no fury like a woman scorned," then you'll be able to imagine the depths of depravity that a witch could cause with a curse uttered just before she dies. This is especially true in a case where the death is by drowning, when the witch in question has been been strapped to a stool that is held underwater in a duckpond.
The carpet delivered to Master Porringer, mere days after the murder of the girl with "something of the goblin in her face," came all the way from London and appeared, at first glance, to be beautiful and a fine addition to any home. However, thanks to the curse cast by the dying witch, the carpet held the plague among its fibers. Soon, five people who came into contact with the carpet were desperately ill, coughing blood and with blackened skin. Within a week, most residents of the village had the plague, and the bodies of all those who died were thrown into a "plague pit" constructed just outside town.
The witch, who later reluctantly reveals to Bod that her name is Liza Hempstock, explains that thanks to the delivery of this very special carpet, everyone who watched her "get drownded and burned" was killed.
After hearing Liza's story, and the tale of Master Porringer's carpet that was a harbinger of the plague, Bod finds that he wants to make Liza smile and therefore decides that he will find her a headstone.