The Feanorians were elves in the elder days of Middle Earth who swore an oath and fled the doom which the Valar laid upon them. They are almost entirely unmentioned in the Lord of the Rings, partly because Tolkien lost the final version of the Lay of Earendil, and sent Unwin's an earlier version. Christopher Tolkien subsequently found this, and published it in volume two of the History of the Lord of the Rings.