King Mark, Tristan, and Iseult
Plenty of people think that this popular medieval tale might have actually influenced the Arthur, Lancelot, and Guinevere story. It's a convincing argument, given that it sounds mighty familiar.
In this myth, King Mark's trusted nephew and awesome knight, Tristan, is sent to Ireland to bring Iseult back as a bride for Mark. Unfortunately, Iseult and Tristan accidentally drink a love potion on the way back to England and fall in love. Their secret passion ends up causing all kinds of problems, and in many versions, it leads to the deaths of all three.