The story
Sāvitrī chose Satyavān knowing what the sage Nārada had told her: that he had one year to live. She married him anyway, kept count of the days, and fasted for three before the last one.
When Yama came for him she followed. Yama told her to turn back; she kept walking and kept talking, and he offered her boons to be rid of her — anything but her husband's life. She asked for her father-in-law's sight, his kingdom, sons for her own father, and then a hundred sons for herself. Yama granted it, and she pointed out that she could not have sons without a husband. He returned Satyavān.
The story is in the Mahābhārata, told to Yudhiṣṭhira, and it is an argument rather than a miracle: she does not plead, she reasons Yama into a corner he agreed to walk into.
The banyan is in it because Satyavān died under one and was revived under it. The tree lives for centuries and sends down roots that become trunks, so it stands for a life that does not end where it seems to.
