The Pandavas
Unfortunately Satyavati's sons die young and her grandson Pandu ascends the throne as his elder brother Dhritarashtra is blind.
Pandu is however cursed by a sage (whom he accidentally kills while he is in union with his wife, mistaking their moans of pleasure to the sound of a deer) that he can never engage in sexual congress with any woman.
He retires to the forest along with his two wives. Using a magical spell to summon the gods Dharma, Vayu, and Indra, his elder queen Kunti gives birth to three sons Yudhishthira, Bhima, and Arjuna through their respective "fathers".
His younger queen, Madri bears the twins Nakula and Sahadeva through the Ashwini twins. Pandu and Madri cannot resist temptation, and die in the forest and Kunti returns to Hastinapura with her sons.
The rivalry between the Pandavas and the Kauravas start from childhood itself. Dhritarashtra’s sons, the Kauravas, led by the eldest Duryodhana, detest their cousins.
However, they were the favorite of their teacher Drona and (the Pandavas) grow up to be exceptional. Each one of the Pandavas is said to have one exceptional strength or virtue - Yudhishthira is the most virtuous, Arjuna the bravest warrior, Bhima the strongest, Nakula the most handsome and Sahadeva the most learned.
When the princes come of age, a tournament is held to display the strength and specialities of the princes of Hastinapur. When Arjuna was hailed as a master of archery, a young man challenges him for a duel.
He declares his name is Karna, and he was the son of the charioteer. When asked to prove that he is of royal birth, which is the criterion for joining the tournament, Duryodhana, spotting a potential ally, jumps over to his side and gives his kingdom of Anga.
Karna is forever grateful for this act. Because of this, he becomes Duryodhana's closest friend and plays a crucial role in the war.