1.)Before Dhronacharya named him Karn he was known as Vasusena, he was the king of Anga.(present day Bihar and jharkhand)
2.) Karn was the only warrior believed to be able to defeat Arjun in battle, Karn’s Digvijaya Yatra, a campaign in which he conquered all kings in every direction of the world, was instrumental in establishing Duryodhana as an emperor.
3.)Karn was the son of Surya and Kunti, so technically he too was a pandav which he knew before the war started but he did not switch sides even though he was offered the entire kingdom of hastinapur by Krishna if he fights for pandav as pandav.
4.)Karn fought against misfortune throughout his life and kept his word under all circumstances. Karna is often quoted for his sacrifice, courage, charity, valour, and selflessness.
5.)Parashuram at a point declared Karn to be equal to himself in the art of warfare and archery.Parashuram accused Karn of stealing knowledge, and laid curse upon Karna that he would forget all the knowledge required to wield the Bramhastra in the moment of need as he was not a Brahmin.
6.)But Parshuram rewarded Karn’s diligence, he gave him his personal celestial weapon Bhargavastra which which no one else possessed. Repenting over a curse made in anger, and in order to nullify said curse, Parashuram gave Karna his personal bow Vijay to be ever victorious in battle and blessed Karna with greatness.
7.) Karn helped Duryodhan marry the process of Kalinga by defeating rest of the suitors. Many legendary rulers like Shishupal, Jarasandh, Bhishmaka, Vakra, Kapotaroman, Nila, Rukmi, Sringa, Asoka, Satadhanwan etc.
8.) Karn took an oath that anyone who approached him with a request, when he worshipped the sun, would not leave empty-handed.Karn was invincible in battle and could not be killed as long as he had his Kavach and Kundal so Indra approached Karn in an attempt to take it away from him. Karn was warned by Surya that this would happen but Karn thanked Surya and explained that he was bound by his word and could not send anyone from his door empty-handed. When Indra approached Karn in the form of a Brahmin beggar and asked his Kavach and Kundal as alms, Karn revealed that he knew the Brahmin’s true identity but assured that he would never turn anyone away. Cutting the armor and earrings off his body, Karna handed them to Indra.
9.) Even without his Kavach and Kundal Karn was invincible and Krishna knew when Karn knows about his real identity it will emotionally weaken him and Kunti can manipulate Karn. As the war approached, Kunti met Karna and in desperation to keep her children alive asked Karna to join the Pandavas. But Karn rejected the offer of Kunti, Knowing that Karna will fight against Arjuna with a motive to kill, Kunti extracted couple of promises from Karna that he will not kill any of the Pandavas except Arjun and against Arjun he will not use a same celestial weapon twice. This particular request by Kunti as suggested by Lord Krishna led to the death of Karn in the war as he did not use Nagastra and Rudrastra twice against Arjuna. Lord Krishna knew that Lord Parashuram gifted entire celestial weaponry to Karn and Arjuna didnt have any counter to many of these weapon.
10.)Bhishma was commander-in-chief of the Kaurava army. But giving the reason that Karn had humiliated Draupadi and disrespected their shared guru, Parashuram, Bhishma refused to take him in the Kaurava army. Secretly, he knew of Karn’s lineage and didn’t want Karn to fight his own brothers.
11.) Surya Offered his invincible chariot and his charioteer Aruna to Karn, just like Lord Indra gave his chariot to Arjun. The chariot of Surya was brilliant as the sun, yoked with 7 horses of different colors and only a person with divine vision could look into it. Confident in his own skills, Karn rejected this offer, saying he didn’t want to be remembered as a person who depended upon others strength to gain victory, indirectly referring to Arjun who depended upon Lord Krishna and Indra.
12.)Krishna Warned Arjun calling Karn to be the foremost of the heroes.According to Krishna, Karn had all the qualities of the five Padavas – the righteousness of Yudhistir, strength of Bhim, archery skills of Arjun, handsomeness of Nakul and wisdom of sahadev.
13.)On the sixteenth day, he fought with all the Pandava brothers but Arjun, defeated them all in direct combat. So violent and offensive were Karn’s attacks towards Arjun that Arjun’s defenses soon crumbled before it. Karn then moved to nagastra, the same celestial weapon that was used by indrajit against Rama in Ramayana. Lord Krishna realized that only miracles can save his ward Arjuna from death and Krishna saved Arjuna from certain death by lowering their chariot and also caused the Sun to set prematurely.
14.) Karn used Bhargavastra against the Pandav army and killed hundreds of thousands of men, steeds, elephants and destroyed countless chariot. Krishna and Arjuna had to run away from the battle field in order to escape from that weapon.
15.) Arjun returned to battle field after Karn withdrew bhargavastra To witness this great battle between the greatest of all the heroes,Devas, celestial Rishis, Apsaras, Gandarva and all the beings in the higher realms and lower realms like the asuras appeared and assembled to witness the battle. Both Pandava and Kauvarava forces stopped their fight and assembled on there respective sides to witness the battle.
16.) Karn cut the string of Arjuna’s bow many times. As the battle intensified, Arjun pushed back Karna’s chariot 10 steps backward every time by the energy of the arrows, but Karn was only able to push Arjuna’s chariot 2 steps backward. Seeing this Lord Krishna praised Karn and admired the skill of Karn. When questioned by Arjuna, Krishna said it is impossible for any human ever to push his chariot backwards because the chariot of Arjuna contains both Hanuman and Krishna, thus holding the entire weight of the universe. Even to shake the chariot is an impossible task, and Karn is pushing it 2 feet (0.61 m) every time by the energy of his arrows. Krishna said no human ever had or ever will attain this feat as it is impossible even to shake the chariot containing Maha Vishnu.
17.)When Karn’s chariot wheel was trapped in the mud as a result of the curse he had received earlier from a Brahmin. He still defended himself, but at the crucial moment, forgot the incantations to invoke Bramhastra, as a result of his guru Parashurama’s curse. Karn got down from his chariot to free the wheel and asked Arjuna to pause, reminding him of the etiquette of war. But Krishna spurred Arjun to attack Karn against the rules of engagement of the war, and the enraged Arjuna attacked Karn while he was trying to lift his sunken wheel with the Anjalika weapon, decapitated the weaponless Karna, who was still trying to lift the sunken chariot wheel. It was later revealed that Karn could be killed only when all the 2 curses acted together upon him, and this made Krishna employ deceit to kill Karna.
18.)The Mahabharata mentions that after his death in the Kurukshetra war, Karna’s soul ascended to Suryalok, and along with his sons and attained the state of a god.
19.)As Karn lay dying on the battlefield, his father Surya and Indra fall into a debate as to who among their sons is superior. They decide to test Karn’s generosity and appear before him as Brahmins asking for alms. Karn told them that at this point he had nothing to give them while one of the Brahmins remarked that he has some gold in his teeth. Realising this, Karn promptly took a stone and broke his tooth, handing them over to the Brahmins, thus proving his superiority.
20.) Also when Arjuna used the Anjalika astra on the weaponless Karn, Krishna found that Karn was still alive even though seriously wounded. Krishna found that Dharma-Devata, the Goddess responsible for protecting Dharma, was guarding Karn from death and resisting every arrows sent by Arjuna because of the massive good merit Karna earned by giving charity during his lifetime. Krishna said wherever Dharma is present there is victory and this time Dharma was with the side of Karna. So Krishna went down from his chariot and appeared as a Brahmin and asked for Karn’s punya or merit, to Him as charity. Karn gave his entire merits as charity to the Brahmin in the form of his blood and once Karn gifted his life’s merit to him, Krishna rewarded Karn with the view of Krishna’s Vishwaroopa and Dharma-Devata disappeared. Then Krishna went back to his chariot and asked Arjuna to take the kill shot on Karna.
Even though he was slayed there was no warrior ever like Karn. Even the god had to kill him with deceit.