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The Ebon Prince Brotherhood
Remismund, a Champion of Recollection, writes:
This mortal cult of Five Days Darkness celebrated a version of the Day of Noon at Night, a festival long removed from its commemorative roots. In the cult litany, called The Reprimand of Noon and Evening, Five Days Darkness was called The Fivefold Ebon Prince, and in their festivals, the cult darkened the skies with kites and smoke, as though an eclipse were obscuring the sun. They believed that this ritualistic behavior strengthens their Prince, who they hoped would one day join the Unconquered Sun in the pantheon of Celestial Incarnae.
My research indicates that the Brotherhood's Reprimand is in fact a fragment of a much larger book, now presumed lost, which is a scientific treatise on mystical and mundane prayer techniques, spiritual etiquette, and other ways of manipulating the world of gods. A production of the Proskemiological University, it is one of many such documents.
It is noted that the quaesitor Mnemon Garel took exception to the existence of the cult, considering it a dangerous solar religion that should be wiped out lest it grow and threaten the Immaculate faith. He took efforts to destroy it, and very nearly did so, but a few cult assassins survived his inquisition and put an end to any further inquiries from him.