Beren And Luthien

Essential to the story, and never changed, is the fate that shadowed the love of Beren and Lúthien: for Beren was a mortal man, but Lúthien was an immortal Elf. Her father, a great Elvish lord, in deep opposition to Beren, imposed on him an impossible task that he must perform before he might wed Lúthien.

This is the kernel of the legend; and it leads to the supremely heroic attempt of Beren and Lúthien together to rob the greatest of all evil beings, Melkor, called Morgoth, the Black Enemy, of a Silmaril.