What are some lesser known stories about Karna?

 on 16th day war, Karna had an edge over Arjuna, after Dushasan's killing. We know that Karna overpowered Arjuna on 17th day of Kurukshetra war. He promised Duryodhan that he will finish Arjuna. He informed his mother too that he will kill only Arjuna. Karna has multiple weapons and mighty Vijaya bow in his hand. Even he got couple of chances to kill Arjuna. But Karna didn't do so. What prevented him?

There is a story that happened during 16th day post the war. Karna had a dream in which he envisioned his guru and asked him to take back the curse he had placed years back. However, Guru rejected his request due to following reasons. In order to protect Arjuna and Dharma, Krishna sought the help of his previous avatar. The avatar Parasurama explained to Karna that if he killed Arjuna, Duryodhana and chaos would ensue. Parashurama asked Karna to accept death and asked him to die at the hands of Arjuna, so that the world might live in peace.

Karna protested to this idea but Parasurama asked this as his Guru Dakshina. Previously, Karna never had a chance to pay his Guru Dakshina; Parasurama never asked for it either. Severely, Parasurama reminds Karna that whatever he became or attained in his life is due to the knowledge he gained from his guru. Karna accepted his guru's words and promised he would never kill Arjuna. Pleased by this offering, the grateful Parashurama blessed Karna with immortal glory and everlasting fame after his death. Thus, Karna never took the kill shot on Arjuna, even though he often had the opportunity to do so.

When Karna's car got stuck in mud, Karna got down from his chariot to free the wheel and he asked Arjuna to wait. Because the Kshatriya dharma says so. But Krishna asks Arjuna to shoot. He reminds of Karna's sin acts when he supported Duryodhan. Arjuna became more furious to kill Karna even when he is unarmed. In this critical situation, Karna didn't give up, he took his bow and fought with arjuna from the ground. Karna defended himself and invoked Rudraastra, hitting Arjuna on his chest. Swooning, Arjuna lost his grip on his Gandiva, which fell down from his hand for the first time. In this situation Karna didn't try to kill Arjuna as per the promise he made to his Guru, but wanted to prove his skills and ability since his meeting with Arjuna from the tournament. Following the rules of engagement of war, Karna did not try to kill the unconscious Arjuna but instead tried to utilize the time in extracting the wheels of his chariot. Arjuna recovered and using the Anjalika weapon, decapitated the weaponless Karna, who was still trying to lift the sunken chariot wheel.