In Hindu mythology, an astra was a supernatural weapon, presided over by a specific deity. To summon or use an astra required knowledge of a specific incantation/invocation, when armed. The deity invoked would then endow the weapon, making it impossible to counter through regular means. Specific conditions existed involving the usage of astras, the violation of which could be fatal. Because of the power involved, the knowledge involving an astra was passed from a teacher to a pupil by word of mouth alone, and only following the establishment of the student's character. Certain astras had to be handed down from the deity involved directly, knowledge of the incantation being insufficient.
Astras come into importance mainly in the Ramayana and Mahabharata, where they are used in the great battles described in each epic. They are depicted as used by archers such as Rama, Arjuna, Bheeshma etc. The astras were generally invoked into arrows, although they could potentially be used with anythingAshwatthama invoked an astra using a blade of grass as his weapon