Complex sentence: A "cista mystica" (secret casket) is a sacred object thought to have Egyptian origin, usually made of reeds (papyrus) and used in the Khoiak ritual of Denderah.
Split into simpler sentences: A "cista mystica" (secret casket) is a sacred object thought to have originated in ancient Egypt. It is usually made of reeds (papyrus) and used in the Khoiak ritual of Denderah.