The Darkhold is the item which controls the nature of magic on the world. It is rumored to take the form of either a book or a sword, and to exist in a multiplanar fashion. It is also rumored to take different forms on different worlds. The nature of magic has changed abruptly several times in the last few centuries, and the operative phrase tends to be a fatalistic "Someone must be messing with the Darkhold!" when this occurs.