Shalya was the king of the Madra kingdom. He gave his sister Maadri to Paandu in marriage. That makes him a maternal uncle to the Paandavas.
He was an excellent mace fighter of his days. Only Bheema, Balaraama and Duryodhana were considered his equals in that genre of fighting .
When the Paandavas and the Kouravas were making preparations and accumulating allies for the war , Shalya's natural selection was the Paandavas' side . But Duryodhana best understood the difference between Shalya' presence and absence on his side . He came up with a brain-wave and ordered his servants to make high class arrangements for sojourning on the way for Shalya and his army all along the route of his travel . Those arrangements were done as ordered and Duryodhana himself was in the group of the workers guised as one.
Shalya was immensely pleased with the comforts offered which he thought were arranged by the Paandavas. He exclaimed in joy, " He who arranged all this for me is entitled to seek anything of his choice from me ! He shall present himself before me . "
Duryodhana got what he wanted and disclosed himself. " Most honoured Sir , It is I who made all this for you . Please keep your word and grant me my wish . "
Shalya gave him liberty to ask and he solicited that Shalya may fight on his behalf. Shalya
agreed and went on to the Paandavas to inform of those developments . Yudhishthira too was happy if Shalya was to keep his word but he requested him to take care of Arjuna in case Shalya would be given the duty of Karna's charioteer in the war. Shalya consented for that also and joined Duryodhana.
As was foreseen by Yudhishthira, Duryodhana convinced Shalya to drive the chariot of Karna in the war against Arjuna on the Seventeenth day. Shalya agreed on the condition that he should be allowed a free tounge. In the war, he disparaged Karna bitterly by saying time and again that Karna was not an equal to Arjuna. In fact, he could disturb Karna psychologically to the extent of losing concentration and composition in fighting. His great Naaga asthra ( serpent-missile ) too missed the target . Karna finally fell but Shalya fully kept his word given to his dear nephew Yudhishthira.
After the fall of Karna, Shalya was made the chief of army and he fought in such a way that not only the Kouravas but the Paandavas too felt that he was going to achive Duryodhana's goal. But , Yudhishthira himself personally lead a comprehensive attack against him and saw him fall with a deadly weapon Shakthi.
Shalya was an example of meaningless commitment and unwarranted doublecrossing . Many critics of the story hold that he was not a man of right stand, since his discouraging of Karna was immoral. Maybe, great men too are not free from vice, after all . Even Yudhishthira himself was derided for personally killing his own uncle who helped him a lot . Again, this is an example how the personal traits and selfish requirements override the commomnly honoured principles , even in the case of esteem characters. In fact , isn't it such highlighting of human traits that hold Mahaabhaaratham high even now?
In any case, Shalya's role earned an immortal saying for us " Shalya-saarathya" , meaning ' Shalya's drivership ' which is referred to a contribution which is seemingly positive but negative in reality . Shalya could do more harm to Karna by his nagging disparagements than what Krishna himself could do by disclosing his relationship with the Paandavas at the most unsuitable time. He was considered one of the six factors of Karna's death. ( Arjuna, Krishna, Kunthi, Indra and the Earth - that held the wheel of his car - were the other five . )
The life of the old man who devouted his body and soul to two opposite sides couldn't have had a better end.