From the wiki article [1]
An athame or athamé is a ceremonial double-edged dagger, one of several magical tools used in neopagan religions and various Witchcraft traditions. It is variously pronounced /ˈæ.θə.meɪ/, /ə.ˈθeɪ.miː/, etc. A black-handled knife called an arthame appears in certain versions of the Key of Solomon, a grimoire originating in the Middle Ages.