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Shalya, king of Madra kingdom and the maternal uncle of the twin Pandavas Nakul and Sahadev was tricked into joining the Kauravas by Duryodhana prior to the Kurukhsetra war.
Upon getting news of the impending war between the Pandavas and Kauravas, Shalya, along with his army of Madra set out to join the Pandavas in battle. But in the middle of his journey, he came across a grand arrangement of food and refreshments waiting for him and his entourage. Assuming that Yudhishthira has arranged all of these, Shalya readily accepted the same. The food was delicious and the entertainment was exquisite and Shalya was extremely pleased with the arrangements. He decided to reward the caretaker and ordered his presence promising any boon he desired. Shalya was surprised and taken aback when he found that it was none other than Duryodhana who had made all these grand arrangements to trick him. But at this point, after promising a boon, Shalya could not go back on his word. Duryodhana requested for Shalya's assistance on his side of the war to which Shalya had to accede.
Although, the great warrior Shalya was tricked into joining sides with the Kauravas, it acted as a boon to the Pandavas in the later stage of the war. Shalya was remorseful for not being able to join the Pandavas. For this reason, when Duryodhana, at Karna's behest, requested Shalya to be Karna's charioteer in the battle with Arjuna he agreed to do so. But his real intention was to help the Pandavas in this guise. Karna was a gallant warrior with a Lion's spirit which needed to be deprecated before he engaged Arjuna. As Karna's charioteer, Shalya started to dishearten him by chanting Arjuna's greatness and trying to prove Karna inferior to him. Shalya even compared Arjuna to a Swan and Karna to a crow who survived on the leftovers of a householder. After some time, Karna started to become disheartened and his will to fight diminished. In this way, Shalya broke the spirit of Karna and reduced his prowess even before he fought Arjuna in that fateful battle.
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