In the Mahabharata, Ambalika is the youngest of three daughters of King Darmamuka of Kasi. With her older sisters Amba and Ambalika, she was abducted from her swayamvari by Bhima.
Bhima gave Ambika and Ambika to King Vicitravirya for his wives. But the king died before his wives had any sons. The Queen Mother, Satyavati, persuaded her oldest and illegitimate son, the sage Vyasa, to beget children on the widowed queens.
Vyasa was frightening and dirty -- apparently sages didn't wash much -- and Ambalika turned pale with fright at his approach. Because she showed fear, her son Pandu was born with a pale skin and red hair.