In an exclusive granted to comicbook.com, Warner Brothers Home Entertainment have offered an exclusive deleted scene and clip that details some potentially ominous story developments for Supergirl and other members of the “Arrowverse.”
From the Arrow complete seventh season set, the deleted scene in question features John Deegan and Supergirl. According to the site:
In the deleted scene…Supergirl finds herself at Arkham Asylum and in possession of the Book of Destiny. After seeing her up close, John Deegan decides that it is she, and not The Flash, who truly personifies power. He vanishes a moment later, suggesting that this is the moment he gets the idea to rewrite reality with himself as Superman.