When Sri Krishna decided to end his stay on earth, it was
time for Balarama, his elder brother, too to depart. How Balarama died is mentioned in the Srimad Bhagavata Purana.
When Dwapara Yuga was coming to an end, Sri Krishna decided
to put an end to his avatar. Soon Dharma (righteousness) started disappearing
and it was replaced by Adharma.
Yadavas, who were into merrymaking and wanton lifestyle,
dressed up Samba as a pregnant woman and decided to mock the Rishis. They
approached the rishis and asked them whether the woman would give birth to a
girl or boy.
The Rishis could not tolerate the behavior of the Yadavas
and cursed them that he would give birth to an iron pillar, which would destroy
the Yadava clan.
Shamba soon gave birth to an iron pillar. King Ugrasena
asked the iron pillar to be powdered and thrown into the sea.
But the powder reached the shores and turned into iron reeds
and which eventually led to the death of the Yadavas.
Balarama who witnessed the end of the Yadava clan ended his life
by his yogic powers at Prabhasa in Saurashtra (modern day Gujarat). (This is mentioned in the Srimad Bhagavata Purana).