Fëanor's Silmarils form a central theme of "The Silmarillion" as the human and elvish characters battle with the forces of evil for their possession. After the Dark Lord Morgoth steals the Silmarils, Fëanor in response swore an oath which all seven of his sons also proclaimed, vowing to fight anyone and everyone—whether Elf, Man, Maia, or Vala—who withheld the Silmarils, swearing upon Ilúvatar himself, and invoking the chief of the Valar and the mountains of Valinor themselves as witnesses. This became known as the Oath of Fëanor and, later, the cause for great tragedy on Middle-earth, particularly among his seven male children.