Ambika was the daughter of King of Kashi and wife of Vichitravirya, King of Hastinapur.
Along with her sisters Amba and Ambalika, she was taken by force by Bhishma from their Swayamvara. (Bhishma challenged the assembled Kings and Princes and defeated them.) He presented them to Satyavati for marriage to Vichitravirya.
After Vichitravirya's death his mother Satyavati sent for her first born, Rishi Veda Vyasa. According to his mother's wishes, both the wives of Vichitravirya to grant them each a son. When Vyasa visited Ambika, she saw his very dreadful and forbidding appearance with burning eyes. In her frightened state, she closed her eyes and dared not open them. Hence her son, Dhritarashtra, the father of the Kauravas, was born blind.
After Dhritarashtra's birth, when Satyavati requested Vyasa to visit Ambika for the 2nd time, she dared not go and sent her maid instead. So the maid gave birth to a son Vidura, who was raised as a brother of Dhritarashtra and Pandu.