Kadru Curses Her Sons and Brahma Approves
To win her wager and avoid slavery, Kadru orders her one thousand snake sons to disguise the white horse's tail as black. When they refuse to aid her deception, she curses them all to die in a future fire sacrifice. The god Brahma hears her and, for the welfare of all creatures, approves the curse.
Kadru had made her wager, but she knew the truth: the divine horse's tail was white. To avoid becoming Vinata's slave, she needed to change reality. She turned to her one thousand sons, the snakes.
Wishing to deceive, she asked them to become like hairs as black as collyrium and speedily cover the horse's tail. It was a simple request from a mother: use your power to help me win.
Her sons refused. They would not do what she had asked.
Her reaction was immediate and terrible. She cursed them. "In the snake-sacrifice of the royal and wise sage Janamejaya, of the Pandava lineage, the fire will consume all of you." She condemned her entire progeny to death in a future holocaust, the Sarpa Satra.
The grandfather Brahma himself heard this extremely cruel curse. The snakes had virulent poison, excessive strength, and were mighty in prowess. They had a tendency to bite, and they had greatly multiplied. Driven by destiny, and bearing in mind the welfare of all creatures, Brahma approved the curse. The other gods agreed.
But even in sanctioning destruction, a countermeasure was provided. For the welfare of all creatures, to counter their virulent poison, Brahma bestowed on the great-souled sage Kashyapa the knowledge of neutralizing poison. The fate of the snakes was sealed, but a means of survival against their venom was granted to the world.