Sage Sthulakesha once found a baby on his doorstep, he named the girl Pramadvara and raised her.
Pramadvara wanted to marry Ruru, she was bitten by a snake before the wedding and died. Ruru was desperate when he heard a voice saying that he could bring Pramadvara back to life. The wedding took place, in Ruru the hatred of snakes remained, each one he killed.
One day he met the (male) snake Dundubha, when he wanted to kill him, he explained to him that he belongs to a harmless species and does not harm people. He was the cursed sage Sahasrapat, who once frightened his friend Khagama with the dummy of a snake and was cursed by him to become one until he met Ruru, who would free him from the curse.
Read the story of Ruru here