Bhishma means He of the terrible oath, referring to his vow of lifelong celibacy. Originally named Devavratha, he became known as Bhishma after he took the bhishana pratigya ('terrible oath') " the vow of lifelong celibacy and of service to whoever sat on the throne of his father (the throne of Hastinapur). He took this oath so that his father, Shantanu could marry a fisherwoman Satyavati " Satyvati's father had refused to give his daughter's hand to Shantanu on the grounds that his daughter's children would never be rulers as Shantanu already had a son (Devratha). This made Shantanu despondent and upon discovering the reason for his father's despondency,[2] Devavratha sought out the girl's father and promised him 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 Devavratha gave up his claim to the throne, his (Devavratha's) children would still claim the throne. Devavratha then took the vow of lifelong celibacy, thus sacrificing his 'crown-prince' title and denying himself the pleasures of conjugal love. This gave him immediate recognition among the gods and his father granted him the boon of Ichcha Mrityu (control over his own death " he could choose the time of his death, but he was not immortal, which would have been an even more severe curse and cause of suffering).
There was a lot of criticism of King Shantanu from his subjects as to why he removed Bhishma from the title of the crown prince (kings didn't have the power to remove a prince from the throne just as Bhishma could not deprive his unborn son from the throne so he decided to never have one)as he was so capable, and if by any chance he did see some fault in him (Bhishma),what did he see in the prince who is not yet born (Satyavati's son). Hearing this, Bhishma said it was his decision and his father should not be blamed as Shantanu had never promised anything to Satyavati's father. The clan Guru then asked who would be held responsible if the future crown prince isn't capable enough. Bhishma then took another vow that he would always see his father's image in whoever sits on the King's throne and will serve him.
In the process of finding a bride for his half-brother the young king Vichitravirya, Bhishma cleverly abducted princesses Amba, Ambika and Ambalika of Kashi (Varanasi) from the assemblage of suitors at their swayamvara. Salwa, the ruler of Saubala, and Amba (the eldest princess) were in love. Upon reaching Hastinapura, Amba confided in Bhishma that she wished to wed Salva. Bhishma then sent her back to Salva who turned her down as it was humiliating for a man to accept a woman who had been so long in the company of another man. Actually he had been embarrassed because Bhishma had defeated him very badly in the fight; he had been no match against the stronger Bhishma. She then naturally approached Bhishma for marriage who refused her, citing his oath. Amba, humiliated and enraged beyond measure, vowed to avenge herself against Bhishma even if it meant being reborn over and over again.
Bhishma abducting princesses Amba, Ambika and Ambalika from the assemblage of suitors at their swayamvara.
Legend has it that at her maternal grandfather's suggestion Amba sought refuge with Parasurama who ordered Bhishma to marry Amba. Bhishma politely refused saying that he was ready to give up his life at the command of his 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, Parasurama was on foot. Bhishma requested Parasurama to also take a chariot and armor so that Bhishma would not have an unfair advantage. Parasurama blessed Bhishma with the power of divine vision and asked him to look again. When Bhishma looked at his guru with the divine eyesight, he saw the Earth as Parasurama's chariot, the four Vedas as the horses, the Upanishads as the reins, Vayu (wind) as the Charioteer and the Vedic goddesses Gayatri, Savitri and Saraswati as the armor. Bhishma got down from the chariot and sought the blessings of Parashurama to protect his dharma, along with permission to battle against his teacher. Parasurama was pleased and said to Bhishma that if he had not behaved in this manner, Parasurama would have cursed him, for it is the duty of warriors who fight against elders not to abandon the traditions of humility and respect for elders. Parashurama blessed him and advised him to protect his dharma of brahmacharya as Parasurama himself must fight to fulfill his dharma of fighting to uphold his word as given to Amba. They fought for 23 days without conclusion " Parasurama was chiranjeevi (immortal) and Bhishma had a boon that let him choose the time of his death.
One version is, that on the 22nd night, Bhishma prayed to his ancestors to help him end the battle. His ancestors gave him a weapon named Prashwapastra which he knew from his previous birth as Prabhasa (one of Ashta Vasus) but forgot it in his present birth as Bhishma. This weapon 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. However the weapon was never used as devas warned him from the sky that "If He uses this weapon it would be a great insult towards his Guru." Pitrs then appeared and obstructed the chariot of Parashurama, forbidding him from fighting any longer. The spirit of Parashurama's father, Jamadagni and his grandfather, Rucheeka, spoke to him:
O son, never again engage in battle with Bhishma or any other Kshatriya. Heroism and courage in battle are the qualities of a Kshatriya, and study of the Vedas and the practice of austerities are the wealth of the Brahmans. Previously you took up weapons to protect the Brahmans, but this is not the case now. Let this battle with Bhishma be your last. O son of the Bhrigu race, it is not possible to defeat Bhishma.
"Mahabharata 188:5[3]
In the end, the Gods showered praise on Bhishma, and he sought the blessing of Parashurama as his guru. The avatar then acknowledged that his former student was truly invincible, telling Amba:
Using even the very best of weapons I have not been able to obtain any advantage over Bhishma, that foremost of all wielders of weapons! I have exerted now to the best of my power and might. Seek the protection of Bhishma himself, thou hast no other refuge now.
"Mahabharata 189:1[3]
However, Amba refused to listen to Parashurama's advice and left angrily declaring that she will achieve her objective by asceticism. Her predicament unchanged, did severe penance to please Lord Shiva (as he is most easily pleased and could give anything to anyone). Lord Shiva assured her that she would be born as a woman named Shikandi in her next birth (and still she would recall her past) and could be instrumental in Bhishma's death, thus satisfying her vow.[4]