The Wood Relief Work above (Karna Trying to Lift the Wheel of the Chariot in the Battlefield of Kurkshetra in Mahabharata) depicts a very important day of the Battle of Kurukshetra from the epic Mahabharata.
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Karna was one of the central characters of the Mahabharta and possibly one of the most powerful warriors. He was the son of The Sun God (Surya Dev) and Kunti. He was born before Kunti got married to Pandu and being an unwed mother, Kunti had to set him afloat in River Ganga. Karna fought the battle for the Kauravas and against the Pandavas.
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Karna though extremely powerful, had been cursed on three separate occasions. Once by his Guru (Teacher) Parashuram who cursed him (since Karna had lied to him about his caste) that the moment when he needs his knowledge the most he would not be able to recall anything. Then by a Brahmin farmer who cursed him (since Karna had accidentaly killed a helpless cow) that he too will be killed when he will be most helpless. And Finally by Bhoomi Devi/Mother Earth. Once a little girl was crying because all her Ghee had fallen on the ground and her step mother would scold her. Karna was passing by and to help the little child he took the soil filled Ghee in his hand and squeezed it so hard that the Ghee got extracted. But this offended Bhoomi Devi and she cursed Karna that during a crucial time in his life even she would use her force and trap him in soil.
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The scene here is a part of the Day 16-17 of the War. Karna had defeated four out of five of the Pandava brothers, Yudhisthira, Bhima, Nakula and Sahedeva. Now the war was on between Arjuna and Karna. Though both of them were equally brave and talented but Karna being the son of Surya Dev had a slight edge over Arjuna.
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During the battle between the two, Karna's Chariot Wheel sank and got stuck in the loose soil as cursed by Bhoomi Devi.. Thus as per the rules of the battle, Karna requested Arjuna to pause as he descended from his chariot to remove the wheel (as exactly depicted in the scene above). At this point Krishna told Arjuna to kill Karna. Initially Arjuna was very hesitant to do so since Karna was helpless at that point and also he would be disobeying the rules of the battle. But when Krishna reminded Arjuna about the fact that Karna and other Kauravas had violated all rules while killing Arjuna's son Abhimanyu, he was motivated to attack Karna who was removing the wheel and kill him.
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This was a very crucial episode in the battle of Kurukshetra because without the death of the invincible Karna, the Pandavas would have never been able to defeat the Kauravas and win the battle, and the victory of Good over Evil would not have been possible.