"The Battle of Evermore" is a parallel to "The Battle of the Pelennor Fields," the final battle of the War of the Ring, taken place before the Gates of Gondor. (The Return of the King Ch. 6, p.114-125)
The Queen of Light took her bow and then she turned to go.
The Prince of Peace embraced the gloom and walked the night
I assume that the Queen of Light is Eowyn, the daughter of King Theoden, who slays the Black captain. The Prince of Peace is Prince Imrahil of Dol Amroth who fights valiantly and returns from battle unscathed to reign over Gondor until Aragorn takes his throne. (The Return of the King Ch. 7, p. 121; 137-40) alone.
...Side by side we wait the might of the darkest of them all...
I hear the horses' thunder down in the valley below.
As the battle rages, the Riders of Rohan and the Men of Gondor must fight 8 of the Ringwraiths in hand-to-hand combat as well as Sauran's other dark legions, but worse than these is the Black Captain, fallen king of Angmar, their ultimate threat.
The sky is filled with good and bad that mortals never know.
...Oh well, the night is long, the beads of time pass slow,
Tired eyes on the sunrise waiting for the Eastern glow
The pain of war cannot exceed the woe of aftermath.
The drums will shake the castle wall.
The Ringwraiths riding black. Ride on.
No comfort has the fire at night that lights the face so cold
"The good and bad that mortals never know" are the magics of Gandalf and the horror of the wraiths mounted on their mumakil. At this point, nearly half of both sides had been slaughtered. As for the aftermath, thousands of men died on those fields, including the wraith king and King Theoden of Rohan. Those fortunate enough to see morning were left the responsibility of gathering the dead in mass funeral pyres. The rest is rather self-explanitory. (The Silmarillion p. 285; The Return of the King p. 116-117)
The magic runes are writ in gold to bring the balance back.
This last line is, perhaps, the most important of them all. It may be taken with several meanings, one being that Gandlaf's runes are written in gold to swing back the balance to good. However, in The Silmarillion, the Valar join forces with the Elves and Men of Good to overthrow Morgoth and sentence him to the Halls of Mandos forever. Although his evil influence is mimmicked by those like Sauron who remained faithful to him, the influence of evil is checked by the Elves. Later, Sauron is also defeated and held captive. During this time many wonderful items are created by Dwarves and Elves, and Sauron , discovering these, loathes them. Feigning to have turned from good to evil he befriends a group of ring-makers so that they will teach him their craft . Three rings are made by the Elves, seven for the Dwarven Kings, and nine for "the mortal men doomed to die," and finally Sauron's ring, the One, inscribed in hideous runes to bind all the rings to his will; thus equaling the balance of good and evil. By the time the events in The Lord of the Ringsoccur, the balance has shifted drastically toward evil. While the Battle of the Pelennor Fields is taking place, Frodo and Sam are venturing up the Orodruin to destroy the Ring to once again "bring the balance back." (The Silmarillion p. 284-304; The Return of the King Ch. 1, p. 177) Note: The Silmarillion was not published until 1977 and is used here only to provide further information on the topic addressed by "The Battle of Evermore."