Sage Kashyap had many wives. Two of them were Kadru and Vinta. One day Kadru asked Vinta a question regarding Uchchaihsravas (Indra's horse). Kadru asked, "Tell me sister, of what colour Uchchaihsravas is?"
Vinta replied, "It is white."
Kadru said, "Its tail is black."
But Vinta was of the opinion that the horse was completely white. Then Kadru made a bet. She said, "One who loses will become the slave of the winner." Vinta agreed.
The two went home and decided to check the horse's tail's color the next day. In the mean time, Kadru ordered her sons (snakes) to transform themselves into black hairs and cover the tail of Uchchaihsravas. Her sons refused to do so. Getting anrgy, Kadru cursed them that they would be killed in a snake sacrifice. Lord Brahma heard this curse. He thought that there were too many snakes and more will come in future. And snakes are poisonous. So it is good for other creatures if there is a way to control their population. Lord Brahma thought that Kadru's curse was a good way of checking the population of snakes. Therefore, Lord Brahma approved the curse of Kadru.
After hearing the curse from their mother, the snakes consulted among themselves and decided that they should do as their mother said. They took the form of black hairs and covered the tail of Uchchaihsravas.
Next day, Vinta and Kadru saw the horse. The horse's tail looked black. So, Vinta accepted defeat and became Kadru's slave.
Garud was born from Vinta. He noticed that his mother always had to obey the orders of Kadru. One day he asked Vinta the reason for this. Vinta told him the whole story. Garud went to the snakes, "Tell me how my mother will be freed from slavery." The snakes said, "Bring to us amrit." Kadru agreed to the same.
Garud told his mother Vinta that he was going to bring amrit. Vinta explained to him where he will find amrit. She also described how well amrit was protected with heavy security.
Garud brought amrit and his mother was freed from the curse.
Will post more detail tomorrow.