Shakuni, Gandhari’s brother, hated Bhisma and wanted to avenge against him by destroying the Kuru Dynasty. He hated Bhisma because of 2 reasons, as mentioned in different narratives.
Firstly, Shakuni loved his sister dearly. Gandhari, had always been afraid of darkness, but when she married Dhritarashtra, she blindfolded herself to share his disability.
Secondly, Shakuni saw his brothers and his father die in front of him. Thus, he hated Bhisma for ruining his sister’s life and for killing his father.
After Saubala’s death, Shakuni was released upon his father’s dying request. Shakuni also had a set of magical dice, which obeyed his orders. These dices were made from his father’s bones, upon his father’s request.
Shakuni was the King of Gandhara but stayed in his sister’s marital home, Hastinapur. When questioned, he would say it was because he loved his sister.