The battle between Bhishma and Parshurama
Bhishma (son of Shantanu and Ganga) was a great archer and a warrior. He is known for his vow of celibacy and skills in the Mahabharata. He had the task of finding a bride for his half-brother, Vichitravirya. He abducted princesses Amba, Ambika and Ambalika of Kashi (Varanasi) at their swayamvara. Salwa, the ruler of Saubala, and Amba (the eldest princess) were committed to each other. When Amba confided in Bhishma that she wished to wed Salwa. Knowing this, He sent her back to Salwa who turned her down as it was humiliating for him to accept a woman who had been so long in the company of another man. She then naturally approached Bhishma and demanded him to marry her, which he refused, citing his vow. Amba, humiliated and enraged beyond measure, vowed to avenge herself against Bhishma.
Preparation of the battle at Kurukshetra
Legend has it that at her maternal grandfather’s suggestion Amba sought refuge with Parshurama, who ordered Bhishma to marry Amba. Parshurama (sixth incarnation of Lord Vishnu) was the guru of Bhishma. He politely refused saying that he was ready to give up his life at the command of his teacher but he could not break his promise. Upon the refusal, Parshurama called him for a fight at Kurukshetra. At the battleground, while he was on a chariot, Parshurama was on foot. Bhishma requested Parasurama to also take a chariot and armour so that Bhishma would not have an unfair advantage. Parshurama 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 Parshurama’s chariot, the four Vedas as the horses, the upanishads as the reins, Vayu (wind) as the charioteer and the Vedic goddesses Gayatri, Savitri & Saraswati as the armour.
The battle between the incarnation of Lord Vishnu and Bhishma
Bhishma got down from the chariot and sought the blessings of Parshurama to protect his dharma, along with the permission to battle against his teacher. Parshurama was pleased and said to Bhishma that if he had not behaved in this manner, Parshurama would have cursed him, for it is the duty of warriors who fight against elders to not abandon the traditions of humility and respect for elders. Parshurama blessed him and advised him to protect his dharma of brahmacharya as Parshurama himself must fight to fulfil his dharma of fighting to uphold his word as given to Amba. They fought for 23 days without conclusion — Parshurama was chiranjeevi (immortal) and Bhishma had a boon that let him choose the time of his death. Two versions exist about how their battle came to and end.
End of the battle
As per one, On the 22nd night, Bhishma prayed to his ancestors to help him end the battle. His ancestors gave him a weapon which was not known to Parshurama . They told him that it would put Parshurama 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 Parshurama will come back to his sense and that shall bring the end to war. However the weapon was never used as Bhishma walked out of the war.
As per the other version, on the 23rd day, Bhishma summoned the infallible celestial weapon(astra) Prashvapastra, the method of using which was known to him and him alone. Neither did a counter-attack exist, nor was a defense against it known to Parshurama . As Bhishma mounted the astra on his bow, a divine voice guided Bhishma not to fire the weapon as its use would lead to the humiliation of Bhishma’s guru(Parasurama himself). Bhishma refrained from using the weapon that would have brought him certain victory. Upon witnessing this, Parshurama was overcome with adulation for his disciple and proclaimed Bhishma as the victor.
Parshurama and Amba
Parshurama thus told Amba that he could not win over Bhishma and gave her the boon of “mahakal shiva”. Amba did penance to please Lord Shiva. Shiva gave her the boon that she would be instrumental in the death of Bhishma. Amba would later be reborn as the eunuch prince Shikhandi in the household of king Drupada.