The Book of Ages is a book that records history as it goes. Whoever writes in it has the power to alter reality. The Demon Sorcerers are afraid of its power.
Shendu sought the Book to change his and his Demon brethren's fates. He possessed Jackie Chan to make his way to Australia, where he begun writing some changes. He created a world where demons rule the world and humans are their ever loyal slaves. Jade Chan is able to escape the changes by accidentally ripping off her page in history.
Because the Book's location is well guarded and deemed No Man's Land, Jade assembles the J-Team, banishing four demons while doing so. Shendu becomes aware about Jade's work when he finds her still existing Moose watch and the remaining demons go to stop the J-Team. Jade decides to even the odds by using the book to empower the J-Team; Jackie grows to 20 feet tall; Tohru becomes a superhero; Viper is cybernetically augmented; Paco makes El Toro the same size but he and Jade constantly fight over making their hero bigger until Uncle tells them to write that he banishes the Demons. Once they are sealed, Jade then replaces the missing page with her name on it, restoring history to normal.
As the book writes all history as it goes, one can simply change a page, thus changing history or empowering anyone through unexplainable means. Also, if someone gets a piece of their part in history, they remain immune to the changes.