The Art of J. R. R. Tolkien's Lord of the Rings--the proem

Is it fair to examine poetry intended as a mythic part of as a larger prose work as poetry? Probably not, but such effects should exist to one extent or another. 

The first line has lovely assonance and consonance--the n's and s's and long e's. The second does the work of bringing together the first two lines by not only continuing the above, but also by adding the "or" sound. Moreover, we are given a repetitive grammatical structure of a "[number] for"--a pattern echoed through two more lines.[And to contrast with the \lines that begin with "One."]

Aside from rhyme and iambic rhythm (a rhythm that it didn't start with, anyway), one has to question strange arrangements like "in their halls of stone." Why move "stone" from the more natural "stone halls" to "halls of stone"? It's actually a brilliant move, highlighting the stone--the hardness and coldness, not just of the halls, but of the dwarves, of the rings and the ring bearers--all bound to the one.

"Under the sky" may not add much to eleven-kings unless there are others who don't live under the sky.

The fifth line breaks the pattern with a line that creates foreboding "In the Land of Mordor where the Shadows lie."

Before the fifth line is repeated in the eighth, we have anaphora of "One Ring to"--including an interior anaphora, which does exactly what the words claim: "bind them." There is rhyme, too, although I'm not sure this is an actual form--A B A B, A C C A

While the final line reintroduces us to the darkness, it's not clear that the repetition adds anything new--certainly nothing more than it's first appearance. Perhaps, then, it is best to consider it as a bit of folklore unearthed in the the mythos of this realm--one designed to set mood and demonstrate the power of the ring.