The magnificent Bhishma who fought bravely in the extraordinary Mahabharata war on the Kaurava side, was then defeated at the hands of Arjuna on Tenth day. Considering the truth that he was blessed with death as per his will, he accepted his death in the course of the war but waited for the auspicious time of Uttarayana of solar to die. On the time whilst his death was near, he was granted a boon by way of Lord Krishna. let us know about it in brief.
After Yudhisthira was crowned as the King of Hastinapur, he took the responsibilities of the entire state along with his people and assigned roles to his brothers Bhima, Arjuna, Nakula, Sahadeva, and others within the kingdom. Then he went to Lord Krishna and asked him about his wellbeing. Lord Krishna was in deep meditation therefore he did not answer to Yudhisthira at that time. After a while, Lord mentioned to him that, he was thinking about the amazing Bhishma; the notable disciple of Lord Parashurama in his thoughts, who was praying to Lord Krishna on his death bed of arrows at that moment. Lord Krishna then elaborated the glories of Bhishma to Yudhisthira and asked him to visit his Pitamaha earlier than his death to get the tremendous knowledge from him regarding the greatest religion and duties of a King and Dharmas related to Varna, Ashrama, and Brahmins. Yudhisthira too agreed with him by saying that, he would be obliged to find out about these from him and it would be divine for his Pitamaha to have the sight of Lord Krishna; The Adideva, and the Brahmamayi Nidhi before he shut his eyes forever.
It was the afternoon of Magha month, Shukla Paksha and Ashtami Tithi. Bhishma was surrounded by many brahmins alongside Krishna Dvaipayana Vyasa there. He kept his mind devotedly in Paramatma only. On the opposite side, Lord Krishna could understand the Bhakti yoga of Bhishma. Four chariots were arranged. First One was for Lord Krishna and Satyaki, second for King Yudhisthira and Arjuna, third one for Bhima, Nakula and Sahadeva and the fourth one for Kripacharya, Yuyutsu and Sanjaya. They soon reached Kurukshetra wherein many courageous warriors had died earlier in the course of the Mahabharata war. Lord Krishna went near Bhishma and asked him about his pain, praised for his excellence, and then requested him to liberate Yudhisthira from mourning over the lifeless warriors via advising him the Dharma specified for the four varnas and Ashramas in line with Shastra, Arthas as according to history, Puranas, Dharma as consistent with Vedas and the Sanatana dharma for Yoga and Sankhya.
Bhishma with folding hands responded to Lord Krishna that, “Narayana! I am pleased to listen to this from you. Well, what am I able to say in front of you considering the truth that the whole thing originated from you only. You're the supreme Lord. My thoughts are not working for the severe pain I experience in my body due to the arrows pierced into it. It's far similar to the pain of being burnt or poisoned. Therefore, I'm unable to speak anything. My power has nearly left my body and the soul is inclined to depart it. Because of the suffering and weakness, my mind is in confusedness. I express regret for my incapability to advise anything. Forgive me. I only live because of you. Even Guru Brihaspati can not advise such as you. How can an ordinary person like me can speak when the Sanatana Purusa and the Karta of the whole universe himself is right here.”
Lord Krishna said, “Bhishmaji! Your statements are reasonable. I gladly provide you with a boon for the ache you are having because of the arrows struck for your body. From now on you wont have any pain, unconsciousness, sickness, burning of the body, any weakness for staying without meals and water. Your wisdom will radiate to your Antahkarana (consciousness) and your mind will now not continue to be dull and will stay in Sattva guna simplest. Your intelligence related to Dharma and Artha will continue to grow with success only. Along with your Divya drishti or divine sight, you'll be able to see Udbhija, Svedaja, Andaja and Jarayuja. Similar to the way fishes are seen inside the clearwater, you will able to see all of the human beings fall into the delusion of Materialistic world.”
After that Vyasa along with all of the sages worshipped Lord Krishna with the mantras of Rig, Yajur and Sama Vedas. Divine flowers showered from the sky. They all took farewell from Bhishma that day and next day early morning Pandavas along with Satyaki and Lord Krishna reached close to Bhishma to hear related Dharma. Narada ji was present there. He instructed all of the Pandavas to clear their concerns regarding Dharma. However, Yudhisthira couldn't query Bhishma out of grief and shame. Lord Krishna asked Bhishma about his state of mind for giving advice and whether or not he has been soothed from the suffering.
Bhishma replied to him that, “Vrishninandan! due to you, all my sufferings are already gone. I can absolutely see the past, present and future. As a result of your boon, I'm able to observe Dharma that is referred in Vedas and Upanishads. My vision is now brightened and I recognize all the religions of the country, caste, human beings, four Ashramas and of a King. While meditating on you, I have now emerged with excellent power much like a young man. By way of your mercy, I will now definitely give advice to Yudhisthira.”