The entire story of Shakuni, Shakuni was in every sense of the word the main instigator of the Mahabharatha.
Dhritharashtra was a wise but a blind king and Ghandhari was his loyal wife who had blindfolded her eyes intentionally. She chose not to see a world which her husband could not see. Ghandhari's astrological signs showed that her first husband would be on his death bed soon and so they got her married to a goat and then they killed the goat. This was a secret known to the Ghandhara family, to which she belonged to before marriage. A few years laters Dhritharashtra got to know about this and he imprisoned the entire Ghandhara family and the king Subala and his 100 sons and treated them very badly.
Every day only one a handful of rice was given to each of them to eat. As days passed one by one started to die of hunger. King Subala realized that this was not how their dynasty was going to end. By now the amount of anger on Dhritharashtra had quadrupled and so he came up with this plan of every one in prison sacrificing their food and handing it over to one person and make that person strong and be responsible for the downfall of Dhritharashtra . The youngest and wittiest son was Shakuni and everyday all would offer their food to him. He could not bear the sight of his father and brothers dying of hunger everyday.
When Subala realized he had no more energy to survive, he called Dhritharashtra and requested him to be considerate on this poor kid and allow him to be free and also promised that he will always be like a guardian to his sons. (Meanwhile Dhritharashtra had 100 sons and a daughter.) He finally pitied his father-in-law and agreed to this. Subala contented with his reply breathed his last. But before dying he told Shakuni to make use of his thigh/back bone and create a powerful magic dice which spins the requested number. He wanted this to be the reason for the downfall of Dhritharashtra and his dynasty. And Subala also broke the leg of Shakuni so that he would remember the injustice done and know his purpose and seek vengeance.
Shakuni as promised, was the guardian of the 100 Kaurava sons and being with them he gained their confidence and instilled all wicked thoughts into everyone. He was the responsible for the Kurukshetra war, which he did by provoking Dhuryodhana against the Pandavas which led to the victory of latter thus fulfilling his fathers wish.