It eventually reveals that the diary is a hoax created by Batman in an effort to have the JSA apprehend Per Degaton at a future time that Batman believed he would not be alive for.
tries without success to alter the course of history for his own benefit, including thwarting the course of ancient historical events like the Battle of Arbela between Greece and Persia in 331 B.C., thus halting technological development and allowing him and his henchmen to conquer an America of medieval-levels of technology with modern weapons which he had left in specially-prepared caves, safe from the alterations to the time stream.
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For example, he provides the revelation of the gruesome punishment that Atom Smasher will suffer at the hands of the Spectre, the breaking down of the relation between Stargirl and Captain Marvel; only the intervention of Daniel Hall prevents Degaton from telling Hector Hall details of his eventual passing.