Thanks @Tusky and @Jubal! If it's lore you're looking for, author Timothy Ferguson runs a
podcast and blog where he's been posting about his research for this book. He's really gone above and beyond!
Quite late in the project I had the idea to frame each creature with a sensationalized headline like you might find in a sixteenth-century chapbook or pamphlet. So the tagline for the Knocker comes from a narrator who is ... less than perfectly reliable.
Don't worry, the text of each creature is presented in a sincere and straightfoward manner. It's true that knockers can play pranks on miners who fail to give offerings, so that's "wrath" the tagline is referring to.