The Book of Souls, also known as the Book of Destiny, is the book where everything that is and ever will be is written. Destiny of the Endless was its keeper for the longest time, although it can be read by anyone if such is their true desire.
Although the identity of its maker is unknown, Destiny has been its guardian and only reader since the Dawn of Time. Chained to his right wrist, Destiny carried the Book everywhere, protecting it from beings like the Luck Lords who have coveted the Book for millennia.
The Lords of Luck
At one point after untold eons, Destiny realized there were four beings who couldn't be seen by the Book -which should be impossible- and whose actions were sporadically rewriting whole pages, with the effect of making the time-space continuum unstable.
Destiny decided the Book wasn't safe with him anymore, but it could be repaired by the same men who were destroying it. So he broke the chain and took steps to place it in their possession.
Unfortunately, by parting ways with the Book, Destiny gave the Luck Lords one opportunity to get hold of it. In order to offset their machinations, Destiny gave Supergirl information regarding the Book and the four beings who could fix it, information that she passed on Green Lantern and Batman. Batman deduced it had to be the Challengers of the Unknown who were living out of the boundaries of destiny.
In spite of the Luck Lords' machinations, the Challengers were entrusted with the Book and the task to fixing it.
The Book of Destiny
As perusing the Book, the Challengers perceived it was giving clues to stop a villain named Megistus from transforming the whole planet into a giant genetic laboratory. Once Megistus was defeated, the Book turned back to normal and was claimed back by Destiny.
Although the Book can be read by whoever who truly so desires, skimming over its pages is a mind-straining action. Roulette was severely affected, Supergirl remained dazed for a while and quickly forgot what she read, and Adam Strange felt more fatalistic.
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