Isolt (or Isold or Isolde) is the female half of the Tristran and Isolde myth. Her mother was a Queen and a witch. When Tristran showed up and won Isolt--to take her back to be the wife of his uncle, King Mark of Cornwall -- Isolt's mother mixed a serious love-potion and gave it to her daughter to drink with her new husband. She was trying to make them happy, of course.
Isolt's handmaid told her that the potion was poison (so she wouldn't drink it until the proper time). But Isolt hated Tristram so much and SO didn't want to be Mark's wife that she drank it anyway on the boatride back to Cornwall--and so did Tristram. So of course they fell in love.
For more of the story, see Wagner's opera if you like that kind of stuff, or go read the myth; it's everywhere. It's a cool tale.