There was a fisherman who lived in Batak Land. His name was Batara Guru Sahala. When he was angling, he caught a fish. He was surprised to find that fish because the fish could talk and it begged to set it free. He did accordingly.
After getting free, the fish changed into a beautiful woman and Sahala fell in love with her. Sahala asked her to marry him and the woman received him. However, she asked his promised not to tell anyone the secret that she was once a fish. They were very happily married, and got two daughters.
It was a pity on Sahala. One day when he got very angry with his daughters, he forgot his promise and he broke it. He told his daughters that they were the daughters of a fish.
His wife could not forgive him. Suddenly, the earth began to shake and volcanoes started to erupt. The earth cracked and formed a big hole. People said that the hole became Lake Toba.