Gandhari was a princess of the Gandhar kingdom, daughter of King Subala.
So to prevent that, Subala, marries her to a goat as a ritual.
Now, somehow Dhritarashtra, finds out about this and goes mad. He vows to elminate the whole Gandhara line to oblivion.
This puts a completely new definition of who the hero of Mahabharata is. This also tells us, that in life, who we think our hero is, might actually be a villain.
It also underlines the importance of good Background verification.