Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
The epigraph of the whole Lord of the Rings series (which appears at the front of each of the three volumes) is Tolkien's own poem:
Three Rings for the Elven-kings under the sky,
Seven for the Dwarf-lords in their halls of stone,
Nine for the Mortal Men doomed to die,
One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.
As we mentioned in "What’s Up With the Title?" we know that a Ring is going to be pretty important to the plot of these books because it says so right on the cover: the Fellowship of the Ring.
While there may technically be nineteen separate, lesser Rings, the important point is that there is One Ring to rule them all. If the Dark Lord on his dark throne can control the One Ring, he will also be able to bring the Three Rings for the Elven-kings, the Seven for the Dwarf-lords, and the Nine for the Mortal Men under his dominion In the Land of Mordor where the Shadows lie. Ta-da! We have the central plot of the novels already spelled out for us on the first page: basically, the world will end if Sauron gets his One Ring back. So someone (and we’re looking at you, tiny Frodo) has to stop that from happening.
As the series progresses, we find that the Nine [Rings] for the Mortal Men are the tools Sauron uses to corrupt and destroy nine great kings of men, thus creating the Nazgûl, the Ringwraiths, his most terrifying servants.
The Seven for the Dwarf-lords have all been lost or stolen by Sauron by the time the novels begin – most recently, the ring of Thrór, which Thórin's father Thráin loses to Sauron in the dungeons at Dol Goldur in Mirkwood (2.2.231), in events mentioned as early as The Hobbit (see Chapter 1 of the "Chapter Summary" in our Hobbit learning guide).
As for the Three Rings for the Elven-kings, which were not made by Sauron and have been kept safe from him through the Ages of Middle-earth, even they would fall if Sauron got his dirty paws on the One Ring. Only once Sauron is destroyed can the bearers of the Three carry them openly: Galadriel wears white-stoned Nenya; Gandalf, red-stoned Narya the Great; and Elrond, blue-stoned Vilya, "mightiest of the Three" (The Return of the King, 6.9.66). But (spoiler alert!) Elrond predicts truly that, "when the One has gone, the Three will fail" (2.2.237); Gandalf, Elrond, and Galadriel only show their Rings as they are riding to the Grey Havens to sail away from Middle-earth entirely. The epigraph to the series predicts that the One Ring [will] rule them all; this power means that, once the Ruling Ring is gone, the lesser Rings have to fade, too.
Engraved in fiery letters on the One Ring are two lines from this poem in the language of Mordor: "Ash nazg durbatulûk, ash nazg gimbatul, ash nazg thrakatulûk agh burzum-ishi krimpatul" (2.2.107). In other words, One Ring to rule them all, One Ring to find them,/ One Ring to bring them all and in the darkness bind them. So we can see that the poem of the epigraph becomes a touchstone for later chapters, as the story of the One Ring and its importance grows clearer and more detailed.
As we read the epigraph before we continue on to the novels, we already get a sense of dread. This is even before we know (a) who has the Ring, and (b) who the Lord of the Rings of the trilogy title even is. The epigraph to this series is both threatening and grim, hinting at the vast, dark powers and magic that face our humble Hobbits.