An astra in Hindu mythology 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.