The Book of Omens is an ancient magical book, containing the history of the legendary race of the Thundercats. A significant element in the Thundercats universe, the Book of Omens is an example of experiencing "literally" the past history, as a spiritual entity. It is sought either by heroes, like Lion-O, or vilains like Mumm-Ra. The Book of Omens appears in the second Disney vs. Non-Disney Villains War, although in the plot continuity, it contains, inside of the Book, a crystal piece, which is in fact a part of the destroyed Crystal of Darkness, necessary for the Acolytes to recollect it.