Of all the Kaurava and Pandava brothers training under Drona, Arjuna emerged as the most dedicated, hard working and most naturally talented of them all, exceeding even Drona's own son Ashwatthama. Arjuna assiduously served his teacher, who was greatly impressed by his devoted pupil. Arjuna surpassed Drona's expectations in numerous challenges. As a reward, Drona gave Arjuna mantras to invoke the super-powerful divine weapon of Brahma known as Brahmashirshastra, but told Arjuna not to use this invincible weapon against any ordinary warrior.