Athamé.
What is Athamé?
Athamé is a ceremonial blade, generally with a black handle. It is the main ritual implement or magical tool among several used in the religion of Wicca, and is also used in various other neopagan witchcraft traditions. A black-handled knife called an athamé appears in certain versions of the Key of Solomon, a grimoire originating in the Middle Ages.
Main uses of the Athamé.
The athamé's primary use is to channel and direct psychic energy.
The ritual drawing of the boundary of the magic circle – also known as 'casting the circle' – is usually done with either a ritual sword or an athamé, in traditional coven practice.
After the casting, the athamé is the tool traditionally used to invoke the elemental guardians of the four directions (also termed 'calling the quarters'), typically by drawing invoking pentagrams at each quarter.
As a masculine principle, the black-handled athamé is often used in combination with the chalice, as feminine principle, evoking the act of procreation, as a symbol of universal creativity. The athamé represents the magical element of air (or fire depending on a tradition), associated with the Sun and the Horned God; while the chalice represents the magical element of water, associated with the Moon and the Goddess. The union of the two then represents the union of God and Goddess, male and female, sun and moon. The marriage of the Sun and Moon—the union of opposites—is an ancient idea in alchemy; and the hieros gamos or sacred marriage of god and goddess is an even more ancient idea in pagan religions. (For example, in ancient Greece—from whence the term 'hieros gamos' comes.) This rite is done by dipping the athamé into the chalice to bless the wine. This is a symbol of the Great Rite in Wiccan rituals.
Sources: Wikipedia & other.
See also if You wish, or need, ... : Witches' Tools of the Trade; Athamé.