Karna was actually Kunti's son and was also the eldest among all the Pandavas. But only Kunti knew this. The story of Karna's previous birth is described in the Adi Parva of Mahabharata. It also describes why Karna's divine armor and earrings could not be of any use to him when he needed them. This story is very interesting.

Asked for help from Nar-Narayan

According to the story described in the Adi Parva of Mahabharata, a demon named Durdumbha (Dambhodbhava) was giving a tough time to the gods. This demon had a boon that only someone who had done penance for a thousand years could kill him. He also had a boon from the Sun God to give him 100 divine earrings and armor, which would result in the death of anyone who broke them. Troubled by this, all the gods sought refuge in Lord Vishnu. Then Shri Hari suggested that he seek help from Nara and Narayana, who were incarnations of Lord Vishnu. In such a situation, all the gods sought help from Nara and Narayana.

The war lasted for many years

Hearing the plea of ​​the gods, Nara and Narayana decided to fight the demon. First of all, Nara fought the demon and during this time Narayana started doing penance. After fighting for many days, Nara broke the demon's armor, due to which he died. But Nara was revived as a result of Narayana's penance. This sequence continued and in this way Nara-Narayan together broke 99 armors of the demon.

After this, Durdumbh got scared and hid behind the Sun God. Then the Sun God prayed to Nar-Narayan to protect the one who had taken refuge. Then Nar-Narayan told the Sun God that if you protect him, then you will also have to suffer the consequences. Due to your power, this demon will be born with armor and earrings in his next life, but when he will need them, they will be of no use to him.

got this result

According to Nara and Narayana, due to the brilliance of the Sun, the demon was born as Karna in his next life and he had the same divine armor and earrings. But before the war of Mahabharata, Lord Indra took the form of a Brahmin and asked for those armor and earrings from Karna, due to which Karna died at the hands of Arjuna in the war.