Bhishma’s sacrificeShantanu, has a son, Devavrata with Ganga. After Ganga’s death, King Shantanu was depressed. After many years of sorrow, he falls for Satyavati, the daughter of the chief of fisherman. But Shantanu has to promise to make any future son of Satyavati the king upon his death.
To resolve his father's dilemma, Devavrata agrees to relinquish his right to the throne and also takes a vow of lifelong celibacy to guarantee his father's promise. He took a vow, to serve future kings and protect the empire. (Mistake made by him, because of his guilt that he was responsible for mother’s death and father’s sorrow). Satyavati’s greed and ambition her own inadequate sons’ that drove Bhishma insane, inflicting gross injustice upon her step-son Bhishma.
Ambha’s curse & RevengeShantanu has two sons by Satyavati, Chitrangada and Vichitravirya. Chitrangada dies. Bhishma attends the swayamvara of the three princesses Amba, Ambika and Ambalika, uninvited, and abduct them. Ambika and Ambalika are married to Vichitravirya.
Amba, wishes to marry king of Shalva and expecting a baby. Shalva, heavily wounded after fight with Bhishma refuses to marry her. Bhishma, also refuses to marry her. She kills herself, after delivering a baby girl. Before her death, she tells that Bhishma will pay the price for his mistake done to her. Amba’s daughter looked like Amba. She learnt all fighting skills and challenges Bhishma. Bhishma can not even look at her, as she brings out his sin of spoiling the life of a woman. He dies at her hands
Vidhura, the WiseVidhura (mostly the title Vidhura - Wise, remote; distant) because of birth was kept away like a slave. Satyavati ensures that he is kept as a slave or the King’s servant. Vidhura wanted to marry Kunti, but snatched by Pandu. Pandu and Vidhura won Gandhara. Bhishma compelled the King of Gandhar to marry off his daughter, Gandhari to blind prince of Hastinapur and enslaving Sakhuni’s (prince of Gandhar and Gandhari’s broher). During hunting Pandu was fatally injured due to low quality sword. Madri, took blame for his death and dies on his funeral pyre. Dhritarashtra becomes a King. Vidhura suspects Shakuni and Gandhara group behind the plot to kill Pandu.
Kunti raises all Pandu’s children. Vidhura has a soft corner for her and her children. Vidhura protects Pandavas from kauravas (children of Dhritarashtra) from many hurdles and help them to live in exile. Arjuna marries draupathi, a tribal beauty who practice polyandry. So, she marries all brothers. Vidura ensures they get their share of kingdom and live independently at Indraprastha.
Vidhura's ChallengeVidhura used to go for pilgrimage every year. During his absence, kauras arranged a dice game and cheats Dharma putra (or Pandavas). Vidhur gets the news from his spies and rushes back to stop, but late. Vidhura confronts Dhritarastra and challenges that he will rectify the situation by getting back Kingdom for Pandavas.
Vidhura is the best fighter and strategist. As a slave, he has to take a vow, to take care of Dhritarashtra and he will never fight against him or his sons. This prevents Vidhura to fight directly for Pandavas. He breaks his bow and declares he will not take part in combat but be a chariotter for Arjuna and by his strategies, he will destroy Kauravas through pandavas.
Pandavas or Arjuna did not have confidence to win a large kingdom or army with experts like Bhishma, Drona .... Vidhura makes them understand need to fight and slowly builds a samll army to fight Kauravas. With good strategy, Vidhura ensures Pandavas eventually win.
Later in some parts of the story, Vidhura was replaced with Divine Krishna.