At the end of Mahabharata, Duryodhana's, body was lying on the field, scarred and wounded. Gandhari, the blind folded mother or Kauravas and the blind king, Dhritharashtra, along with Sanjaya, (the suta who narrated Bhagavad Gita after Sri Krishna) were surveying the death and destruction caused by the war. The Pandavas along with Sri Krishna arrive at the seen. On being informed of their presence, Gandhari's tears dry up and with uncontrollable anger, shouts at Krishna as to why, in spite of her being his staunch devotee, in spite of Pandavas and Kauravas, being created and sustained by Lord Krishna himself, he lets this debacle happen. Why he couldn't have averted the war itself. On seeing, His all knowing smile even at such a time, she curses the entire Yadava family to be killed in the next 36 years. Sri Krishna grants her curse to come true.
Sri Krishna returns to Dwaraka and takes good care of his subjects. During this time, the Yadavas attain wealth and prosperity. The Yadavas were considered to be quarrelsome people and in the lap of luxury, they loose their morals and humility.
Samba was the son of Sri Krishna and Jambavati. But in character, he was very naughty and frivilous. One day, when the Saptarishis, were passing through the kingdom, Samba and his friends decide to test the powers of the learned Rishis. So Samba dresses up as a pregnant woman and comes to the Rishis. One of his friends asked the Rishis if the child born from the expectant "mother" will be a male or female. The Rishis see through the prank and getting enraged, curses Samba that whatever Samba gives birth to, will end not only him, but the entire race of Yadavas. Frightened, they run away from there.
Soon enough, Samba gets pregnant and with great agony, delivers an iron pestle (a rod used with a mortar to ground spices or grains.) The Yadava's did not want to keep the pestle that was supposed to cause their destruction, in their kingdom. Ugrasena (father of Kamsa), ruler of Dwaraka, orders the pestle to be grounded, made into fine dust and dumped into the ocean. Sri Krishna smiles silently as the scared yadavas rush to carry out the order. The Yadavas reduce the pestle to dust, except for a very hard piece of the pestle. They throw the dust and the single hard piece of pestle into the ocean. Having done this, they come back satisfied that thwarted fate. (Little did they know, they cemented it.)
As time goes by, the Yadavas fall more into merry making ways, become intoxicated and were generally hedonistic. Seeing this, Sri Krishna, asks the Yadavas to visit Prabhas sea (a dip in Prabhas see is considered holy) to wash off their sins. They all reach the sea, but with the finest quality of liquor and meat in spite of Ugrasena's decree that none in Dwaraka would touch liquor. They drink to their hearts content. Then a minor skirmish breaks out between Yuyudhana and Kritivarma. From here it breaks out into a full fledged war in between the Yadavas. In their inebriated rage, they break blades of the eraka grass (a form of bamboo found near Prabhas sea shore) and wield it at each other. The strong eraka grass blades crashed into the skulls and bones of the Yadavas. Soon enough, all the Yadavas were dead except Daruka, Balarama and Sri Krishna. Daruka was asked to convey the events to Vasudeva. After the carnage, Balarama decides to leave the earth. He meditates by the sea, and the sea consumes his body. Sri Krishna sits down next to a shrub ruminating about the turn of events. He is killed by the arrow of a hunter who mistakes the Lord's feet to be an animal hiding behind a bush.
Conclusion:
At the end of Mahabharata, Sri Krishna had prayed to Lord Shiva for a son like him. A son like Shiva. A son with equivalent destructive powers as the Lord of destruction himself. And this son (Samba) was instrumental in destruction of an entire race.
The dust of the pestle, thrown into the sea gets pushed back onto the shores by the waves and grows into a form of hard grass/bamboo near Prabhas Kshetra near present day Gujarat, called Eraka grass. Legend has it that, the grass is as hard as iron. The very fate that the Yadavas tried to run away from became the instrument of their death. The unbroken piece of pestle that was thrown into the sea was swallowed by a fish. The fish was caught by a fisherman, and the metal piece was given to a blacksmith, who makes a deadly arrow out of it. This arrow changes hand with a hunter. The hunter uses the arrow on what he thinks was an animal hiding behind a shrub!
The dust of the pestle, thrown into the sea gets pushed back onto the shores by the waves and grows into a form of hard grass/bamboo near Prabhas Kshetra near present day Gujarat, called Eraka grass. Legend has it that, the grass is as hard as iron. The very fate that the Yadavas tried to run away from became the instrument of their death. The unbroken piece of pestle that was thrown into the sea was swallowed by a fish. The fish was caught by a fisherman, and the metal piece was given to a blacksmith, who makes a deadly arrow out of it. This arrow changes hand with a hunter. The hunter uses the arrow on what he thinks was an animal hiding behind a shrub!
Sri Krishna had granted the wish of Gandhari, his devotee, by designing the destruction of his race and himself!
Just goes to show that the wheel of Karma is applicable on everyone. Everything has a beginning, middle and end, (no matter who it is.) We are here only to be an instrument of Gods Will and we depart after we finish our part. So make sure you play your part well.