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Dhu Dwimmermorc is an ancient and yellowed manuscript which was part of Ciri's curriculum in Ellander while she was studying with Yennefer at the Temple of Melitele. It is described as "famous and terrible, full of menacing etchings". It also has a whole chapter dedicated to omens, according to Yennefer.
The name of the book comes from Elder speech and probably mean "Book of Black Magic".