Early Life and Training
Karna became interested in warfare and approached Drona, teacher of his Kaurava cousins. He was refused on the grounds that he was not of the warrior caste. He then learnt by himself, seeking his immortal father as a guide. However, when he wanted to learn archery, he sought Parashurama, incarnation of Vishnu and teacher to Drona. He knew Parashurama only taught brahmins (those of the highest caste) and so appeared as a Brahmin. Towards the end of his training, Karna offered Parashurama his leg to rest his head on. While Parashurama was asleep, a bee stung Karna. He did not flinch as he did not want to disturb his teacher. When blood flowed from the wound, Parashurama woke up and deduced Karna was not a brahmin. He then laid curse upon Karna that he would forget all the knowledge required to wield the divine weapon Brahmastra, at the moment of his greatest need. Upon Karna's pleading, Parshurama relented and gifted him the celestial weapon Bhargavastra, along with his personal bow, Vijaya, for being a diligent student.
Karna once encountered a child who was crying over her pot of spilt butter. On asking her the reason for her dismay, she stated that she feared that her stepmother would be angry over her carelessness. Refusing to take new butter from Karna, the child insisted that she wanted the same butter. Taking pity on her, Karna took the soil mixed with butter in his fist and squeezed it with all his might, so that the butter dripped back into the pot. During this process, Karna heard the agonized voice of a woman. When he opened his fist, he realized that the voice was that of Prithvi. She chastised Karna for inflicting enormous pain on Mother Earth for the sake of a mere child and cursed him that at a very crucial moment in battle, his chariot wheel would be trapped as tightly as he had held that fistful of soil.
During the War
Karna entered the war on the fifteenth day as commander of the Kaurava forces, alongside Drona, replacing the injured Bhishma. Kunti secretly visited her firstborn son and revealed his parentage. He vowed to spare his brothers should he meet them in battle, save Arjuna. On the seventeenth day, Karna and Arjuna resumed their battle, when Karna’s chariot wheel sank into the loose ground. He still defended himself but also forgot all incantations for divine weapons at the crucial moment, as was cursed by Parshurama. Karna got down from his chariot to free the wheel and asked Arjuna to pause reminding him of battle etiquette. But Krishna (another of Vishnu's incarnations) in turn reminded Arjuna of all the rules that Kauravas had broken in the war and before. Enraged with recollection of all the incidences Arjuna slew Karna.