Bhishma falls on the tenth day of eighteen and does not die until long after the fighting stops, because the boon of choosing his hour lets him wait for the sun to turn north. That interval is where the Mahabharata puts its longest sustained teaching. Yudhishthira, sick with what winning has cost, comes to the dying grandsire and is answered across two entire books, the Shanti Parva and the Anushasana Parva, which together run to something like a quarter of the whole epic.
The material is not devotional in the main. It is government: how a king raises revenue and where the limit is, how law binds a ruler who is also its enforcer, what a king may do when the ordinary rules cannot be kept, which is the doctrine of apaddharma or duty in extremity, and then, at the far end, the paths of release. It is the point at which an epic of a family quarrel turns into a manual for people who have to govern.
Set within it is the Vishnu Sahasranama, the thousand names of Vishnu, given by Bhishma in answer to a question about what is highest, and recited daily by many Vaishnavas who have never read the war books around it. The framing is deliberate. The teaching on power is put in the mouth of a man who held vast authority, used it to keep an oath rather than to prevent a war, and is spending his last weeks explaining, to the winner, what he should have done differently.