Shakuni was the mastermind for the outbreak of Mahabharat. He would always instigate Duryodhan to trouble the Pandavas. Whether it was to poison the sweets eaten by Bhima as a child or the burning down the wax house where the pandavas resided, Shakuni would always have a plan ready in his sleeve.
The dice game was his major plot after which the outbreak of the battle between the Pandavas and Kauravas became inevitable.
The 'dice'y game
The dice game was a result of jealousy, prosperity and insults. When the pandavas were given arid and un prosperous land of Hastinapur, they turned it into a prosperous and enviable city called Indraprashtha. The news about the beauty of the city spread all over. The curiosity to see the city tempted Duryodhana so much that he decided to visit the city personally.
When he went to the palace, he mistook water for a floor, and fell into it. On seeing this Draupadi, the wife of the five Pandavas, burst out laughing and insulted Duryodhana by saying that the son of a blindman can only be a blindman. Enraged, Duryodhana returned to Hastinapur.
Reading his nephew's state of mind, Shakuni plotted a clever plan to strip Pandavas of Indraprastha. He invited the Pandavas to a friendly game of dice against Duryodhana. When the game started he gave Yudhisthir the chance to win a few games. Soon using his skills and the special dices made with his father's thigh bones, he won the game. Yudhisthir in the process lost his kingdom, brother and even placed his wife on stake. Whenever Yudhisthir tried to back out, he would be taunted by Shakuni and his nephews. Thus through this dubious game, Yudhisthir lost everything to his cousins. They were then forced to go on exile for thirteen years in some distant forest. They were ordered to not come under the purview of anyone i.e. they had to leave a life in a society without being recognized.
Shakuni took the side of Kauravas during the Kurukshetra battle. He was killed by Sahadev- one of the pandav brothers in the battlefield.