The Mahabharata war wasn't merely a display of weapons and military might; it was a confluence of religion, emotions, and mysteries, the impact of which remains even today. When the Kauravas and Pandavas faced each other on the battlefield of Kurukshetra, both sides possessed warriors capable of changing the course of the war. One of these was Karna, the son of the Sun—an unparalleled philanthropist and warrior capable of single-handedly taking the lives of the four Pandavas. But a secret promise saved all four brothers and etched an immortal example of magnanimity in the pages of history.  

The Secret Before the Battle and Kunti's Arrival
Just before the start of the Mahabharata war, Kunti, the mother of the Pandavas, arrived to meet Karna. She knew that the armor, earrings, and divine weapons possessed by Karna would make it impossible for her sons to survive. She revealed to Karna the ultimate truth about his birth: that he was not a son of a charioteer, but a boon from the Sun God and her eldest son.  
With tearful eyes, Kunti urged Karna to abandon the Kauravas and join the Pandavas. If he did so, as the eldest son, the throne of Hastinapur and the honor of the five Pandavas would fall to him.  

Karna was moved by the truth from his mother Kunti, but he flatly refused to side with the Pandavas. Karna respectfully told his mother    :

"O Mother! When society had insulted me by calling me a son of a charioteer and ridiculed my talent, only Duryodhana embraced me, made me the king of Angadesh and gave me respect. Today, when the time of war has come, if I betray my friend and come to you, then there will be no one in this world more ungrateful than me."

Although Karna refused to abandon Duryodhana, it was against his nature to return empty-handed to his mother. Karna consoled Kunti with a great promise:    Sparing the lives of the four Pandavas: Karna vowed  that he would not inflict fatal blows on Yudhishthira, Bhima, Nakula, and Sahadeva on the battlefield with his divine weapons, nor would he kill them.  

Main battle only with Arjun: Karna's final battle will be only with Arjun.

Truth of five sons: Karna said to Kunti - "O Mother! Your lap will never remain empty. Even at the end of the war, only five of your sons will be alive. If Arjun is killed, I will remain as your fifth son, and if I am killed, then all your five Pandava sons will be alive."  

During the war, there were many occasions when Karna defeated Yudhishthira, Bhima, Nakula and Sahadeva and made them the target of his arrows. If he wanted, he could have killed them in a moment, but remembering the promise he had made to his mother, he left them alive every time.  

Finally, on the 17th day, Karna and Arjuna fought a battle, where Karna, trapped in a web of fate and curses, attained martyrdom. After the battle, when Yudhishthira learned the truth that Karna was his elder brother, he was overwhelmed with grief and performed Karna's last rites with full brotherly love and respect.  
Karna's sacrifice, the pinnacle of friendship, and his promise to his mother will forever remain immortal in history.  

PC: Inkhabar