Once the eight Vasus visited Vashishta's ashram with their wives, one of the wives took a fancy to Nandini and asked her husband Prabhasa, to steal it from Vashishta. Prabhasa the vasu,stole it with the help of the others, and was cursed by Vashishta to be born in the world of men. The seven Vasus who assist in stealing Nandini have their curse softened to be liberated from their human birth as soon as they are born, but Prabhasa, due to his being instrumental in the theft, is cursed to endure a longer life on the earth, though the curse is softened so that he becomes one of the most illustrious men of his times.
The youngest brother is born as Bhishma the youngest son of Shantanu by his first wife Ganga (the holy River), The other 7, were born as the older siblings of Bhishma, who were drowned by their mother Ganga as soon as they were born, thus fulfilling the softened curse on them.
Shantanu was the 12th king of a line starting from Dushyanta and Bharata, though the vansh (family or progeny) is said to have started with Bharata the great.
Bhishma learnt political science from Brihaspati, the guru of the Devas, Vedas and Vedangas from rishi Vasishta, and archery from Parashurama, also known as Bhargava, thus becoming an exceptionally skilled administrator, as well as an undefeatable warrior. His banner in battle was a golden palm tree.
He was known as 'Bhishma Pitamaha' (i.e., Bhishma, the grandfather or grandsire) among the Pandavas and the Kauravas. Bhishma is considered to be one of the greatest examples of a dutibound officer.
'Bhishma' means 'He of the terrible oath', referring to his vow of life-long celibacy. Devavrata became known as Bhishma because he took the bhishan pratigya — the vow of life-long celibacy and of service to whoever sat on the throne of his father (the throne of Hastinapur). This was because when his father Shantanu wanted to marry a fisherwoman Satyavati, her father refused, on the grounds that his daughter's children would never be rulers as Shantanu already had a son (Devavrata). This made Shantanu despondent. To placate Satyavati's father, Devavrata promised that he would never stake a claim to the throne, implying that the child born to Shantanu and Satyavati would become the ruler after Shantanu. At this, Satyavati's father retorted that even if Devavrata gave up his claim to the throne, his (Devavrata's) children would still claim the throne. At this, Devavrata, to make his father happy, took the terrible vow, thus sacrificing his 'crown-prince' title and denying himself the pleasures of intercourse. This gave him immediate recognition among the gods and his father granted him the boon of Ichha Mrityu (control over his own death — he could choose the time of his death, but not, as may be suggested, one of immortality).
Bhishma was a great archer and a warrior of peerless valour and courage. In the process of finding a bride for the young king Vichitravirya (son of Shantanu and Satyavati) for whom he was the regent, Bhishma challenged the assemblage of suitors at the swayamvar of princesses Amba, Ambika and Ambalika of Kashi (Varanasi) and defeated all of them. Unknown to Bhishma, Salwa, the ruler of Saubala was in love with Amba (the eldest princess) who reciprocated his feelings. On the way to Hastinapura with the princesses, Bhishma was confronted by Salwa who challenged him to a battle for the hand of Amba in marriage. After a hard fight, Salwa was vanquished and admitted defeat. Upon reaching Hastinapur Amba confided in Bhishma that she wished to wed Salwa and no other. When Bhishma sent her back to Salwa, the vanquished ruler turned her down in humiliation of losing the combat. Upon being turned down by Vichitravirya too, as a maiden who had loved another man, Amba was incensed at Bhishma, whose interference she perceived as the root cause of her troubles.
Amba took refuge with Parasurama, the guru of bhisma. Parasurama ordered Bhishma to marry Amba. Bhishma politely refused saying that he is ready to leave his life at the command of the teacher but not the promise that he had made. Upon the refusal Parasurama called him for a fight at Kurukshetra.
At the battlegrounds, while Bhishma was on a chariot, he saw his guru on the ground. He requested Parasurama to be equal to him by taking a chariot and Kavacham (armor). Parasurama blessed Bhishma with the power of divine vision and asked him to look again. When Bhishma looked at his guru with the divine eye-sight, he saw the Earth as Parasurama's chariot, the four Vedas as the horses, the Upanishads as the reins, Vayu as the sarathy (Charioteer) and the Vedic goddesses Gayatri, Savitri & Saraswati as the armor.
Bhishma got down from the chariot and sought the blessings of Parasurama to adhere to his dharma. Parasurama told him that if he would not have behaved in this manner Parasurama would have cursed him. Parasurama advised him to fight to protect his dharma of bramacharya and Prasurama would fight to protect his duty towards the word given to Amba.
They fought for 23 days without any result. Parasurama is a chiranjeev or immortal whereas Bhishma had the boon of death at his wish. On the 22nd night, Bhishma prayed to his ancestors to help him to bring the war to an end. His anscestors gave him a weapon which was not known to Parasurama . They told him that it would put Parasurama to sleep in the battlefield. A person who sleeps in the battlefield is considered to be dead as per Vedas. They advised Bhishma to call back the weapon at the end of day after sunset so that Parasurama will come back to his sense and that shall bring the end to war.
On the 23rd day, when Bhishma took the weapon given by his pitru's, a divine voice spoke to him asking not to use the weapon and insult his guru Parasurama and it told Parasurama that he cannot win over Bhishma in the war. But Parasurama said that he cannot go back from the war when Bhishma is still standing against him in the battlefield. Bhishma in respect of his teacher walked away from the combat and allowed a graceful exit for Parasurama.
Parasurama told amba that he could not win over Bhishma and gave her the boon of "mahakal shiva". Amba did penance to please Shiva. Shiva gave the boon that she will be instrumental for the death of Bhishma. Amba would be born as a princess in the house of king Drupada, and as a consequence of another boon would be transformed into Shikhandi (a male) and be the root cause of Bhishma's death.
He is the one who witnessed the Mahābhārata completely from the beginning since the rule of the Shantanu.
In the great battle at Kurukshetra, Bhishma, bound by his oath to serve the ruler of Hastinapura, fought very reluctantly on the side of the Kauravas; nevertheless, he gave it his best effort. At one stage, his impeccable military prowess, combined with Arjuna's disinclination to fight him, almost made Lord Krishna break His vow of not actually fighting in the war. Krishna charged at Bhishma to kill him with a chariot wheel and was welcomed with folded hands by the grandsire. Then Arjuna pleaded with Krishna to stop and reminded him of the vow.
Bhishma was finally grievously wounded on the tenth day of the battle by Arjuna, who hid behind another warrior Shikhandi, and rained arrows on the grandsire. Bhishma knew that Shikhandi was born a woman and to strike a woman he deemed unworthy of the chivalrous. Thus, the warrior did not resist but merely remarked to Dushasana, "These are Arjuna's arrows, they cannot be Shikhandi's because they tear my flesh as a crab's young ones tear their mother's body." Of all of Duryodhana's commander-in-chiefs, Bhishma had held off the inevitable defeat the longest. He was the supreme commander of the Kaurava forces for ten days compared to Drona's five, Karna's two and Salya on the final day. Bhishma fell, his entire body resting on a pincushion of Arjuna's arrows. After that Drona become the Commander-in-Chief of Kaurav army. After his demise Karna replaced him. Soon after this, Karna, who in the face of Bhishma's criticism had sworn his vow to keep out of the Kurukshetra till the withdrawal of Bhishma, approached the grandsire to seek his blessings. Bhishma reveals to Karna that he always knew the truth of the latter's parentage and implored him to persuade Duryodhana (at this point he also told Karna that he had not allowed to fight him under his command as he did not want the real brothers to fight with each other) to end the carnage that had already resulted in such great slaughter. Upon Karna's refusal and insistence to remain true to Duryodhana, he nevertheless received the grandsire's blessing. He lay on the 'bed of arrows' till the end of the battle, and chose to die only after learning that the Pandavas had won, as he was now assured that the throne of Hastinapura was in safe hands. In his last days before he ascended to heaven, he recited to Yudhisthira the famous hymn to Vishnu, the Vishnu sahasranama. Bhishma also admitted he had been wrong to fight for Duryodhana even though he was the king's employee since one's only allegiance is towards righteousness.