The tome will turn anyone reading it into a devout worshiper of Cyric and thus increase Cyric's power. The tome is of great power and once you start reading you cannot willingly stop, and it even works against other deities. The final version of the Cyrinishad was the 398th version, and all the previous authors had suffered greatly for their failure to satisfy Cyric.
The book is made of raven-black leather, embossed with a pattern of small grinning skulls and sunbursts against a twisting, warped background of torture and chaos. The book has golden hinges and clasps, and it is closed with a lock of unbreakable metal.
A skull the size of a child's fist dominates the front cover, the skull is animated by Rinda's blood and is the guardian of the book, preventing it from being destroyed or removed from the mortal realms.
After Mask managed to trick Cyric into leaving the book in his hands it was given to Rinda the Scribe for safekeeping, protected and concealed by Oghma. Later "Gwydion the Quick" joined Rinda as guardian of the book.
They kept it hidden from Cyric and his followers for a long time, until Malik el Sami yn Nasser (who would later become the Seraph of Lies) managed to track them down at Candlekeep. Though both Rinda and Gwydion were slain the Cyrinishad was lost near Candlekeep and has yet to be found.