Vichitravirya, son of Shantanu and Satyavati was the king of Kuru Vamsha. Ambika and Ambalika were his wives. Vichitravirya died of Tuberculosis. He didn't have children who would inherit the kingdom. Sage Vyasa was Satyavati's son from Sage Parashara. She called upon Vyasa to impregnate Ambika and Ambalika so that the vamsha would continue. Vyasa was a tapaswi and had a fierce form. When he united with Ambalika she closed her eyes in fear. Because of this, their son Dhritarashtra was born blind.