Question: When did Karna make his impulsive vow about not participating in the battle until Bhisma was killed?
Answer by Romapada Swami:
This is mentioned in the Udyoga-Parva of Mahabharata. This occurs after the Pandavas have passed their 13th year of exile incognito, and Arjuna has defeated the Kauravas who had attacked Virata’s kingdom. All sides are busy seeking allies for the war, Duryodhana has chosen Krsna’s army and Arjuna has chosen Krsna, etc. The following incident is immediately after Duryodhana has told Dhrtarashtra that he (Duryodhana) is more powerful than even demigods so he will easily defeat the Pandavas.
Karna then said: “Oh King, there is no need for you to feel that you must rely upon your own prowess, for I alone can vanquish all of the Pandavas!”
Bhisma then angrily interrupted Karna by saying, “O Suta’s son, you may be boasting, for anyone can talk like a hero. I can assure you, however, that with the help of Lord Krishna, Arjuna will not only cut down your excessive pride, but your very life as well.”
Karna replied, “O Grandsire, I admit the superiority of Lord Krishna, but since He has vowed not to take up weapons, there is nothing He can do to harm us. O Bhisma, because you have addressed me so harshly, I hereby vow not to fight in the upcoming war until after I have witnessed your death!”
Later in the Udyoga Parva, after Krsna and Kunti have both unsuccessfully tried to persuade Karna to switch sides and both sides are preparing for war, there is another mention of this vow:
Bhisma then said, “O King, I shall accept your proposal, but at the same time I must warn you that since the Pandavas are very dear to me, I will never disregard their welfare. In this world, I can see only one warrior who is equal to me, and that is Arjuna. However, I know that, out of respect, he will never fight with me very fiercely. Similarly, even though I’m capable of easily vanquishing all of the demigods, demons, human beings, and Rakshasas combined, I shall never kill the Pandavas. My dear Duryodhana, I hereby vow to slay at least 10,000 warriors each day, but I must insist upon one condition before accepting the post of commander-in-chief. Either Karna or myself will fight first, but not both of us together, for he has always insulted me while vainly boasting of his own prowess.” At this, Karna assented by saying, “O king, I agree to fight only after Bhisma has been killed, for that was the vow which I had already made earlier.”
