The Glyph Key Is a large metal emblem in the shape of a T1 glyph and the only thing that can open the Compendium of Reproach. Both became lost at some point in history and their recovery marks a turning point in the plot of TDS.
Somehow, the glyph key had made its way to the Sunken Citadel, and was the object of at least two failed excursions made by the Keepers. Although Keeper Rafe managed to collect the key in the latest expedition, he was unable to escape with it and died in the Kurshok library.