Book of Prophecies

The Book of Prophecies (予知書) is a book that appears in Kingdom Hearts χ and Kingdom Hearts Union χ. It is the source of and s, which allow  to harness powers from the future.

Kingdom Hearts χ
By using the power he installed into the No Name Keyblade, the Master of Masters wrote the Book of Prophecies which foretold the events that would lead up to the Keyblade War and what came after. He then placed his Gazing Eye in the Keyblade and gifted it to one of his apprentices, Luxu, in order for it to make and transmit back through time the observations which would compose the Book in the first place. In order to prevent undue temporal paradoxes, he did not give Luxu a copy of the Book. Instead, he gave copies to his five other apprentices, who became the foretellers, with all but Gula's omitting the "Lost Page" (ロストページ) which covered the events of the betrayal of the foretellers, and how the Keyblade War would begin.

After reviewing the manuscript, the foretellers discovered the shocking final entry, which prophesied:

To avert this crisis, the foretellers established Unions to battle the darkness and used the Book of Prophecies to draw power from the future. Player's Chirithy later informs them of its creator's actions so they can understand the source of their power.

According to Gula, the "Lost Page" talks about the one who bears the sigil. Parts of it are revealed by him to Skuld and Player when Ava confronts Luxu outside Daybreak Town.

When the Master of Masters hands Ava a list with the five Dandelion Union leaders that eventually would succeed the foretellers after the Keyblade War, he encircled one of their names red to ensure this one would get a copy of the Book of Prophecies, along with the rulebook all five would receive.

However, Ava reneges on the Master of Masters' orders in hopes of changing fate. She chooses Brain to be a Union leader and gives him a copy of the Book of Prophecies, as he had not been chosen by the Master of Masters to receive it. Before parting with him, she places the memo from the Master of Masters in Brain's copy of the Book of Prophecies, hoping that he would understand it when the time came to decide their future.

Kingdom Hearts Union χ[Cross]
After the conclusion of the Keyblade War, the new Union leaders discover a new species of Dream Eaters that can banish Nightmares from their masters. Brain sends Ephemer, Skuld, and Ventus to collect the necessary ingredients to create them while he and Lauriam stay behind to make the necessary preparations.

Over time, Brain continuously reads through the book, believing that they were still on a destructive path. Brain resolves to be the virus that overwrites their destiny in order to avoid another Keyblade War.

Kingdom Hearts Re:coded
Following their defeat in the Datascape, Maleficent and Pete discuss the ancient Book of Prophecies. According to legend, it is capable of transcribing and conjuring future events, as well as creating entire worlds filled with beings and powers that have yet to exist. Maleficent decides to search for a connection between the Book of Prophecies and the Datascape, and she wonders if the Book is also connected to Jiminy's Journal.

Kingdom Hearts III
Maleficent searches for the Book of Prophecies in every world she can think of, believing the Book is locked away in the black box.

Trivia

 * Thus far, there are seven existing iterations of the Book of Prophecies, the original held by the Master of Masters, the five copies belonging to the Foretellers, and Brain's copy.

Notes and references
Livre des Prophéties Buch der Prophezeiungen Libro de las Profecías