Jara( old age, decay) was an old demoness. Bitter and desperate, scavenging for food once day, she came upon two parts of a baby lying in different parts of a forest.. it was more than she expected, she was about to settle down to make a meal of them, when some curiosity/compassion make her bring the two pieces closer together. A infant started bawling before her. She recognised, from some sign that this was the king's child and took the child across, maybe out of compassion for the king, maybe hoping to also get a reward that would make the hunting and scavenging that was difficult in her old age, un necessary
The king beamed to see his son and named him Jarasandha- joined by Jara. The childless king of Magadha had longed for a son, through an elaborate prayer he had been granted a boon- a mango, bearing the assurance of fertility. The king who loved both his queens equally, gave each of them half a mango. The attendants of each queen had thrown the strange half child born to each, out into the forest.
Jarasandha, the fleeting hope of old age and decay, ruled over the powerful kingdom of Kashi. Feeling the vulnerability within, he made himself stronger without. He built mighty fortifications to his city, he lived as a powerful man, bore children, made alliances. He married his two daughters to Kansa, king of Mathura, that ran along his borders.
Then Kansa was killed by Krishna and a new enmity started. Jarasandha attacked Krishna and Balarama eighteen times, but failed to vanquish them. The eighteenth time thought, he thought he had killed them in the mountains. Krishna meanwhile, created the new city of Dwarka on the sea, and lived there with his citizens. He married eight primary wives and some 6,000 others, representing the kingdom of Bharata thus.
Jarasandha started a yagya for Victories in the Four Directions, he imprisoned some 20,000 rulers who refused to defer to him. At the same time, Yudhisthira, cousin of Krishna, wanted to do the rajasuya yagya in the Khadva forests, in Indraprashtha. But for that, he would need the derference of the neigbouring kings. Krishna's counsellor, Uddhav, told him that Krishna must defeat Jarasandha and free all those kings on Yudhisthira's behalf if he wanted to help his cousin do this yagya. But Jarasanda must be defeated in physical combat.. He also told him that Jarasandha never refused a brahman anything.
So Krishna went, with Arjuna and Bhim, all dressed as Brahmans, to Girivraja, Jarasandha's capital fortified by hills. They challenged Jarasandha to a wrestling match. Jarasandha refused to fight with Krishna who was not a brahman.
He fought with Bhim, they were equals, there seemed no chance of anyone winning. Krishna indicated to Bhim that he must cleave Jarasanda in two.
Bhim lifted Jarasandha up holding him by his legs, cleaved him in two, and threw the two parts far away from each other.
So came to an end an unlikely, tender labour of an old demoness who looked like the deepest part of the wild forest that she wandered in.