'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. 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 (female) and be the root cause of Bhishma's death.

almost correct except for few errors