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The Godskin Prayerbook is one of the many Key Items found throughout Elden Ring.
When brought to either Brother Corhyn or Miriel you can learn new incantations. The Godskin Prayerbook unlocks Black Flame, and Black Flame Blade.
Found in Stormveil Castle. Starting from the Rampart Tower site of grace head out the door and take a running jump onto the rooftop. From there drop down the left side and head down the staircase to a room with rats and a fog door. Use a Stonesword Key to open the fog door, beyond that there are two chests: the one on the right contains the Godskin Prayerbook, and the other has the Godslayer's Seal.