In "Morgoth 's Ring", the first of two companion volumes, Christopher Tolkien describes and documents the later history of "The Silmarillion", from the time when his father turned again to 'the Matter of the Elder Days' after "The Lord of the Rings" was at last achieved. The text of the Annals of Aman, the 'Blessed Land' in the far West, is given in full; while in writings hitherto unknown is seen the nature of the problems that J.R.R.Tolkien explored in his later years, as new and radical ideas, portending upheaval in the old narratives, emerged at the heart of the mythology, and as the destinies of Men and Elves, mortals and immortals, became of central significance, together with a vastly enlarged perception of the evil of Melkor, the Shadow upon Arda.

The second part of this history of the later "Silmarillion" is concerned with developments in the legends of Beleriand after the completion of "The Lord of the Rings".