As the Kauravas were losing the battle, Duryodhana approached Bhishma one night and accused him of not fighting the battle to his full strength because of his affection for the Pandavas. Bhishma, greatly angered, immediately picked up five golden arrows and chanted mantras declaring, “Tomorrow I will kill the Pandavas with these five golden arrows.” Duryodhana, not having faith in Bhishma’s words, asked Bhishma to give him custody of the five imbued golden arrows saying that he would keep them with him and return them the next morning.
Long before, when Pandavas were in living in the forest, Duryodhana once came to the forest to flaunt his material opulence in contrast with the Pandavas who were living in exile due to his wicked usurption of their kingdom, thus displaying his venom and hubris. He placed his camp on the opposite side of a pond where the Pandavas used to stay. Once, while he was taking bath in that pond, the heavenly princes, named Gandharvas, came to take a bath.
Duryodhana could not tolerate this; he had a fight in which the Gandharvas captured him. On the request of Yudhisthir, Arjuna saved Duryodhana and set him free. Duryodhana was ashamed, but being a kshatriya, he asked Arjuna what benediction or boon he would like. Arjuna replied that he would ask for the boon later when he needed it.
It was during that night that Krishna reminded Arjuna of his unsatisfied boon and told him to go to Duryodhana and ask for the five golden arrows. When Arjuna came and asked for the five golden imbued arrows, Duryodhana was shocked, but knowing full well his honour and duty as a kshatriya declared, “I will give you the five golden arrows. But can you please tell me who told you the five golden imbued arrows existed in the first place?” Arjuna replied with a smile, “Who else other than Shri Krishna could have advised me?” Afterwards, Duryodhana honourably kept his word and gave the five golden arrows imbued with a lifetime of austerity and power by Bhishma. Later, Duryodhana broke his customary manner and conceded to himself privately, “All of the rishis say Krishna is the Supreme God, maybe they are right.” Being shocked and convinced by Arjuna’s knowledge of the very existence of the five golden arrows, Duryodhana secretly got on his knees and said a quick prayer to Krishna.
Duryodhana again went to Bhishma, informing him of all that happened and requesting another five golden imbued arrows. To this Bhishma laughed and replied, “That is not possible, child. The will of the Lord is Supreme and undeniable. I have already spent the merits of my lifetime of celibacy and austerity in those five arrows, a potency strong enough to end all the five Pandavas. However, Krishna, being the Supreme Lord has foreseen this, knowing past, present, and future as He alone can, and has thus arranged for you to voluntarily give up the five golden arrows, imbued with the power and potency of my lifelong austerities. I cannot imbue another five arrows, having spent all of the merits of my austerities. However, tomorrow I shall fight like a lion, and this time, either I will kill Arjuna or I will make Lord Krishna break His promise of not picking up any weapons during the war.” On the next day there was an intense battle between Bhishma and Arjuna. Although Arjuna was very powerful, he was no match for Bhishma. Bhishma soon shot arrows which cut Arjuna’s armour and then also his Gandiva bow. Arjuna was helpless before the wrath of the grandsire.
As Bhishma was about to kill Arjuna with his arrows, Lord Krishna threw down the chariot reins and jumped off the chariot onto the battlefield, lifted a chariot wheel and charged Bhishma. Arjuna tried to stop Lord Krishna, but the Lord said, “In order to protect my devotee, I must break my own promise.” However, this is actually a double entendre. Krishna, one who was a perfect Yogi and in control of senses including anger, lifted the Chariot Wheel in order to respect and uphold Bhisma’s promise and word, as the latter promised he would make Krishna yield weapons.
Eventually, however, Arjuna convinced Krishna to return to the chariot and put down the wheel, promising to redouble his determination in the fight. All the while Bhishma stood with folded palms and tears in his eyes, awed by the beauty of the wrath of the Lord and the intense love that God bears for his friends, so great that He is willing to endure the censorship of ignorant foolish men. Later the Lord told Arjuna how he could bring down the old grandsire, through the help of Sikhandhi. Using Sikhandhi as a shield, Arjuna shot arrows at Bhishma, piercing his entire body. Thus, finally, Bhishma gave up the fight, focusing his life force and breath, sealing the wounds, and waiting for the auspicious moment to give up his body.
Its said that once Bhishma fell down pierced with arrows he was feeling thirsty. Duryodhana went to fetch water, but Arjuna shot an arrow at the ground and water came pouring out directly into Bhishmas mouth. Its actually river goddess Ganga coming to satisfy the thirst of her son Bhishma.
Source : Wikipedia