In the Mahabharata, the description of Mandavya Rishi comes. Once the king mistakenly missed justice and cursed his seniors to offer sage Mandavya to the shul. But when the sage's life did not go away for hanging for a long time, the king realized his mistake and took down the sage Mandavya from the shul and apologized for his mistake. 

After this Rishi Mandavya went to meet Yamraj and asked him why I was punished for the false charge. When you were 12 years old, you killed a small insect, due to which you had to face this punishment. 

Then the sage Mandavya said to Yamraj that no one knows the religion and what is unrighteous at the age of 12. Because you have punished me very much for a small crime, so I curse you that you will be born as a maid's son in Shudra Vagina. Due to this curse of Mandavya Rishi, Yamraj had to be born as Vidur.