What is the story behind the birth of Vidura? Was there any curse for him to be born in the mortal world?
When Niyoga was not successful as expected for 2 widows of Vichitravirya, named AmbikA and AmbalikA, the queen Satyavati again asked AmbikA to give a second try. The princess refused this time and sent her Shudra maid. The maid remained calm during the Niyoga and hence beget a very good child, Vidura. However Vidura was born out of a curse from Rishi Mandavya. From Adi Parva:
Some time after, when the oldest of Vichitravirya's widows again had her monthly season, she was solicited by Satyavati to approach Vyasa once again. Possessed of beauty like a daughter of a celestial, the princess refused to do her mother-in-law's bidding, remembering the grim visage and strong odour of the Rishi. She, however, sent unto him, a maid of hers, endued with the beauty of an Apsara and decked with her own ornaments. And when the Vyasa arrived, the maid rose up and saluted him. And she waited upon him respectfully and took her seat near him when asked. And, O king, the great Rishi of rigid vows, was well-pleased with her, and when he rose to go away, he addressed her and said, 'Amiable one, thou shalt no longer be a slave. Thy child also shall be greatly fortunate and virtuous, and the foremost of all intelligent men on earth!' And, O king, the son thus begotten upon her by Krishna-Dwaipayana was afterwards known by the name of Vidura. He was thus the brother of Dhritarashtra and the illustrious Pandu. And Vidura was free from desire and passion and was conversant with the rules of government, and was the god of justice born on earth under the curse of the illustrious Rishi Mandavya.