An earlier version of this oath from around 1925 appears in The History of Middle-earth, volume 3, which says:
Be he friend or foe or foul offspring
of Morgoth Bauglir, be he mortal dark
that in after days on earth shall dwell,
shall no law nor love nor league of Gods,
no might nor mercy, not moveless fate,
defend him for ever from the fierce vengeance
of the sons of Fëanor, whoso seize or steal
or finding keep the fair enchanted
globes of crystal whoso glory dies not,
the Silmarils. We have sworn for ever!
How does Fëanor's Oath haunt the rest of The Silmarillion? Why is the concept of making and/or breaking an oath or vow so important in Tolkien's works?