Brother of Gandhari, who hated Bheeshma for making his sister marry Dhritarashtra, who was blind. According to some folk versions, he was also angry at Dhritarashtra for imprisoning and killing his 100 Brothers. He was the king of the Gandhara kingdom. He is sometimes considered to be the main antagonist of the Mahabharata.
Clever and shrewd, Shakuni used his talents to destroy the Kuru race because of the aforementioned reasons. Shakuni was also a great illusionist. He influenced his nephew, Duryodhana, from his childhood, and led him to hate his cousins, the Pandavas. He advised Duryodhana into using various methods to kill or weaken the Pandavas. That included:
- Advising Duryodhan to poison Bheema twice
- Hiring Purochana with Duryodhan to kill the Pandavas in the House of Wax
- Arranging the game of dice, in which he cheated, which led to Draupadi's humiliation.
- Before the war, he advised Duryodhan to feed Shalya's army, to put Shalya in his debt and therefore make Shalya fight for him.
Family and Early Life
Shakuni was the son of Subala, Gandhara's king, and his mother was Sudarma. He had a sister named Gandhari, who he loved dearly. He was a crafty and devious man. He was the mastermind of the Kurukshetra War. He was a good illusionist. He married Arshi and had 4 children: Uluka, Vrikaasur, Patatri, and Vriprachitti.
The original texts of the Mahabharat personify him as Dwapara Yuga. 
Relation with Hastinapur
Bheeshma was intent on making an heir to the throne of the Kuru Kingdom at Hastinapur, so he approached Subala with a marraige proposal for Gandhari and Dhritarashtra. Shakuni hated the idea of his sister marrying a blind man. He despised Hastinapur even more when he realized that Dhritarashtra was not going to be the king. He was further angered when Gandhari told him that she did not want to marry Dhritarashtra, but Bheeshma gave her no choice. Gandhari blindfolded herself as a silent protest. Seeing this, Dhritarashtra refused to accept Gandhari as his wife, and Shakuni was angered further. So, Shakuni vowed to destroy Hastinapur. 
Plots and Influence
Plot to Poison Bheema
When the young Pandavas and Kauravas are playing, Shakuni made a point to always support Duryodhan in whatever childish argument. Shakuni would turn these small fights into big issues, and whenever the Pandavas beat the Kauravas in the games which they were playing, Duryodhan would get angry.
Plot to burn the Pandavas
Since this plan was foiled, Shakuni kept warmongering, and one day told Duryodhan a plan. They would invite the Pandavas to a palace made of Wax, then they would hire Purochana to set it aflame. The plan worked with success until the Pandavas created an escape route when the found out the plan.
Plot of the Game of Dice
After the incident of the House of Wax, the Pandavas still have no emnity to the Kauravas. The Pandavas conquer the world. Shakuni helps Duryodhan get jealous of the Pandavas' success. He advises Duryodhan to play a game of dice with the Pandavas, as Shakuni has dice made out of his father's bones. So, whenever he rolls the dice, his father, in heaven, can decide what number to land on. So, Shakuni never loses a dice game.
This is a big embarrassment for the Pandavas and now the Pandavas are really angry at the Kauravas, which is what Shakuni wanted. Shakuni also wanted Bheeshma, his main enemy, to get on the wrong side of the Pandavas. Duryodhana orders his brother, Dussasana, to disrobe Draupadi. Draupadi prays to Lord Krishna, who stops Dussasana. This game was very important as it is the main cause of the Kurukshetra War. 
During the war
On the 18th day before war, Duryodhana convinced Shakuni to be the Commander-in-Chief of his army but he preferred Shalya. Shakuni participated in the Kurukshetra War and defeated many warriors.
On the very first day of war, Shakuni, Duryodhana and Dushasana attacked Yudhishthira to kill him but they failed. On the 2nd day, he got defeated by Sahadeva in a Sword Duel and by Satyaki in Archery. On the 4th day, he killed 5 powerful Generals of Magadha. On the 5th day of war, he got defeated by Arjuna in Archery battle and by Dhrishtadyumna in Sword Duel. On the 7th day, he got defeated by Bhima in Gada-Yuddha and by Drupada in Archery.
On the 9th day of war, he got defeated by Yudhishthira in a Spear Combat and by Abhimanyu in Archery. On the 10th day of war, he stopped Shikhandi from reaching Bhishma. He also defeated Shikhandi in a battle of Axe-Fighting on the same day. Shakuni also stabbed him in his stomach. On the 12th day, he slew king Sahadeva of Magadha. Later, he defeated Upapandavas and Virata. He also fought with Dhrishtadyumna two times, firstly he defeated him and secondly he got defeated.
On the 13th day, Shakuni and other Maharathis attacked and killed Abhimanyu. Many of them backstabbed Abhimanyu. On 14th day, he fought with Nakula to save Jayadratha but got defeated. After Jayadratha's death, he planned a Night War. In the night war he defeated Drupada, Virata and Satyaki. On 17th day his son Vrikaasur was killed by Nakula. 
After the Game of Dice episode, Sahadeva had taken an oath to kill Shakuni, the evil mastermind of the Kurukshetra War.
On the 18th day of the battle, the Pandavas attacked Shakuni, Uluka and their army. As Duryodhana and his other brothers rushed to protect their uncle, Bhima stepped in and fought the remaining Kauravas and killed many of them (except Duryodhana). Meanwhile, Nakula killed many prominent Gandharan warriors and the bodyguards of Uluka. Sahadeva fought Shakuni and Uluka. Not long afterwards, Sahadeva killed Uluka. Shakuni became furious and attacked Sahadeva. He broke his chariot and bow, but Sahadeva ascended another chariot and fought Shakuni ferociously. After much attacks and tackles, both of them descended their chariots to settle things in a duel. Sahadeva was then able to smash an axe into Shakuni's forehead, fulfilling his oath.
Shakuni died after the injury. 
- A version, originating from Jain texts, is very popular but this is never mentioned in the Mahabharata. As per the story, Gandhari, being a Mangalik (person whose first spouse was supposed to die soon after the marraige), was married to a tree (some say a goat) prior to her marriage to Dhritarashtra, in order to nullify the defect. On hearing of this, a disgusted Dhritarashtra ordered the extermination of Gandhara's royal line. Shakuni, his father, Subala and Subala's other hundred sons were captured and put in a prison. One grain was served to every person. Knowing that Shakuni was the smartest, Subala and his 99 sons sacrificed their lives and provided their share of grain to Shakuni. Later, they all died and Shakuni vowed to destroy Hastinapura. However, in Mahabharata, Shakuni's brother fought in the Kurukshetra War, proving this to be a popular myth.
- According to another Jain version, that contradicts the Mahabharata, while Shakuni was still a young boy, Gandhara was invaded by a Kuru prince, with (different versions of the story naming the attacker as Pratipa, Shantanu, Devavrata, or Vichitravirya). Shakuni, his father, his brothers, his uncles, and his cousins were imprisoned, with Hastinapur arguing it was to restore dharma to Gandhara, but Shakuni claiming that it was a pure power-grab. When the Gandhara royal family argued that food must be given to prisoners, only one grain of rice is given to each captive. Knowing that Shakuni is the wisest among them (and in some versions the youngest) and most able to take revenge, the prisoners give all their food to Shakuni so that he can survive. Eventually, all of Shakuni's imprisoned family members die. His uncle (or father) begs for mercy and bends the knee to Hastinapur, freeing Shakuni who vows his revenge. Shakuni received a boon from his father that he will be a great politician and will defeat other people with his political genius.
- Shakuni is the personification of Dwapara Yuga.
- There exists a temple in Kollam District, Kerala, which is dedicated to Shakuni. 
- All images used have the link in the captions
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- "Why Gandhari's father got her first marriage with a goat! Click here to know | NewsCrab". www.newscrab.com.
- Ritu (17 October 2013). "Shakuni – the real story". Indian Mythology.
- Suri, Chander Kanta (1992). Shakuni. Books For All.