Krishna had a son called Samban. Jambavati was his mother. He used to live in Dwaraka. One day Vishwamitra, Kanva and Narada were visiting Dwaraka. Some yadava boys decided to have a little fun. They dressed Samban up as a pregnant woman and took him to the maharshis. They asked the sages whether the child will be male or female. The sages saw through the prank and were enraged. They said that Samban would deliver an iron rod and it would cause destruction of the yadavas. True to the words of the sages, Samban delivered an iron rod. Worried about the curse, yadavas made the rod into dust and threw it into the sea. The dust was washed ashore and became grass. The grass, when pulled out, turned to iron rods. Yadavas used these rods to beat each other and ultimately perish.
There is also a legend that one of the pieces of the iron rod that Samban delivered was washed ashore and was picked up by a hunter. He made it into an arrow. It was this arrow which caused Krishna’s death.
At the end of the great war, Gandhari went to the battlefield with the widows and other women. She was heart-broken seeing the bodies of young warriors which lay all around the field. Gandhari, in a state of rage, cursed Krishna and his clan – they were to perish from the face of the earth in 36 years. In Gandhari’s mind Krishna was one who controlled the way events turned out and she felt that he could have avoided the great catastrophe. This curse also came into reality through the ‘pregnant Samban’ episode.