Having recently watched the DVD of the final Harry Potter film, I just noticed some rather awful writing on the back of the penultimate DVD's box. Its summary begins: "Harry, Ron and Hermione set out on their perilous mission to track down and destroy the secret to Voldemort's immortality and destruction — the Horcruxes." It's bad enough if you understand what Horcruxes are, but this sentence makes things especially confusing for those who don't. Why's that?
Well, it's true that Horcruxes are "the secret to Voldemort's immortality" and also that they are "the secret to his destruction", but in completely different senses of being "the secret to" the thing in question. The former sense is one of providing an explanation for why something is the way it is. The second is about assisting someone in bringing about a certain end (and it's a less common use of the word).
As it is, one is forced to interpret these qualities (immortality and destruction) as sharing this "secret" in the same way, which results in nonsense. Is the summary saying that Horcruxes are the secret to how Voldemort "acquired immortality", and also how he (later on?) "acquired destruction"? Great, that means the bad guy is out of the way! Or, wait, is it supposed to mean that Horcruxes are the secret means by which the heroes hope to bring about Voldemort's destruction, and also the means by which they hope to bring about his immortality? Make up your minds, silly wizards!
Anyway, there's a simple fix someone could have applied if they'd noticed this — just add a word like "key" which means the same thing as "secret" in this instance. Also, for the heck of it, I'd remove the repetition of "destroy" and "destruction". Here's my new, improved first sentence:
Harry, Ron and Hermione set out on their perilous mission to find and destroy the secrets of Voldemort's immortality and the keys to defeating him— his Horcruxes.