I just read a great book called Haunt Me Still by Jennifer Lee Carrell. It's the sequel to her previous book, Interred With Their Bones, and probably the second book in a Shakespeare trilogy. This book is much better than the first, and I recommend skipping the first and reading this. It's all about paganism and Macbeth--what more do you want, really? And even though it's a work of fiction, there's much that should be taken seriously. She presents the origins of Macbeth's plot in terms of not only history but of mythology. Shakespeare borrowed quite a lot of his plot's from ancient mythology: both Hamlet and Richard II, for instance, are variations on the story of Osiris from Egyptian mythology, one of the oldest stories in existence. The Lion King, Dune, and, oddly enough, Iron Man 1 are also based on that story.
She also presents a theory about the act without a name that the witches are performing. She links this with the idea of going into the underworld (or death) while still alive and returning transformed, which is the hero myth (see the Pietroasa bowl). This seems to have been the point of the Hellenistic mystery religions, Egyptian New Kingdom mysticism (Amduat), and quite a lot of modern mysticism as well (read the Dalai Lama's description of an esoteric practice in his intro to the Tibetan Book of the Dead).