The child Kunti had placed in a basket was found by Radha, the wife of Adhiratha, a charioteer, who raised Karna as their own son.
Karna was initially trained by Dronacharya alongside the Kuru princes, but Dronacharya refused to teach him the Brahmastra, the divine celestial weapon, as Karna wished to outshine Arjuna. Karna left Dronacharya’s ashram and went to Parashurama.
Since Parashurama taught only Brahmins and Kshatriyas, Karna lied that he was a Brahmin and was trained by Parashurama.
One day, while Parashurama was asleep on Karna’s lap, an insect stung Karna. He endured the pain and did not move in order not to disturb his guru while he slept.
When Parashurama woke up and saw the blood, he questioned Karna, who explained that an insect had stung him.
Realizing that only a Kshatriya could endure such pain, Parashurama grew suspicious, and Karna revealed to him that he was a Suta (the son of a charioteer) and had lied to learn the Brahmastra.
Angered, Parashurama cursed Karna that when he most
needed to use the Brahmastra, he would forget the
mantra to invoke the Brahmastra.