Karna was half brother to the Pandavas and son of Surya, but raised by a Suta charioteer. Karna came to Parashurama after being rejected by Drona because he imparted knowledge only to Kshatriyas, and Karna was, a son of Suta. Karna did not know of his true parentage and hence said he was a Brahmin, and so Parashurama accepted him as his student, giving him knowledge of the powerful Brahmastra weapon. Parashurama trained Karna to such a point that he declared Karna to be equal to himself in the art of warfare and archery.
One day, Parashurama was sleeping with his head resting on the lap of Karna and a scorpion crawled up the leg of the student and bit his thigh. In spite of the pain, Karna neither flinched nor cried so his guru could rest. Warm blood, however, trickled down his leg, waking Parashurama. Convinced that only a Kshatriya could have borne such pain in silence, Parashurama realized the lie of Karna, and cursed his student that his knowledge would fail him when it was most crucial.
Upon Karna's pleading, Parshurama gifted him the
- celestial weapon called Bhargavastra, along with his
- personal bow called Vijaya, for being such a diligent student. 
Years later, during the Kurukshetra war, Karna had a dream in which he envisioned his guru and asked him to take back the curse he had given years back. Parashurama revealed that he had known all along Karna was a Kshatriya, but because he was a worthy student Parashurama had instructed him regardless. The avatar explained to Karna that the Brahmastra had to fail him when he needed it most. If he killed Arjuna, Duryodhana would be king instead of Yudhishthira, and chaos would ensue. Parashurama asked Karna to accept his curse and asked him to die at the hands of Arjuna,so that the world might live in peace. Karna accepted his gurus words and in return the grateful guru, Parashurama blessed Karna with
- immortal glory and
- everlasting fame.
Later during the war, Karna died when shot fatally by Arjun for the sake of his guru's words.