A forbidden key that was guarded by the Eldest Brother of the Brotherhood, but was eventually lost, thus forever sealing the chamber it opened.
Sold by Candelaria for 
400 in Mercy Dreams.
Lore
... I hear his lament. The one who was believed mute from the beginning, since there were witnesses to the First Miracle. The afflicted one who asked for punishment, and had it granted to him. I hear his lament. Even if the Father is no longer with us, even if only his images, large and small, remain in all the sacred places. I hear his lament. It is the Miracle that makes me hear it, this is my punishment as the Eldest Brother and guardian of this congregation whose mystery is the silence of the Father. I hear his lament, and I don't want to hear it anymore. I will have yours cover His. That is my penance.....