In what song does Led Zeppelin sing about "Lord of the Rings"?

Both Page and Plant, in keeping with the times, were readers of J.R.R. Tolkien's The Lord of the Rings. Plant has commented lightly on the possibility of lyrical allusions to LOTR in his music, and even gone so far as to say that he and Jimmy Page would "play music and write songs about the Lord of the Rings." He can even be quoted as saying "After reading Tolkien, I knew I had to move to the country," thus proving how deep an impact the trilogy made on him, as it has to many of us. In his lyrics, particularly those for "Ramble On" and "Battle of Evermore," Plant uses Tolkien-specific terms such as Gollum, Ringwraiths and Mordor. Other, more indirect allusions may be found elsewhere throughout their repertoir.
http://www.ledtolkien.com/

Led Zeppelin is commonly believed to have incorporated "The Lord of the Rings" into their songs, incuding "The Battle of Evermore", "Ramble On", "Misty Mountain Hop", and "Bron-Y-Aur Stomp".
"The Battle of Evermore" is supposed to have related to The Battle of the Pelennor Fields in "The Return of the King". "The magic runes are writ in gold to bring the balance back" is believed to refer to the Elvish writing across the band of the One Ring and the destruction of the Ring. The Prince of Peace, who "embraced the gloom" and "walked the night alone" is believed to be Frodo, Aragorn, or Gandalf, according to different interpretations. Frodo went into the darkness of Mordor with only Samwise Gamgee, and is therefore most commonly believed to be the the Prince of Peace. Aragorn is sometimes believed to be the Prince of Peace because of his lone wanderings on "the path of the dead" to find the Dead Men of Dunharrow. Gandalf's claim to being the Prince of Peace is in his battle with the Balrog in the darkness of the mines of Moria. There is also "the Queen of Light" who "took her bow" referred to in the song. She is commonly believed to be either Galadriel, the luminescent Queen of the elves of Lothlorien, or Éowyn, the beacon of hope for the Rohirrim who took up arms to fight for her people.
https://en.wikipedia.org/wiki/The_Battle_of_Evermore

"Ramble On" also has an impressive amount of reference to "The Lord of the Rings". The opening line of the song, "Leaves are falling all around", is probably a paraphrase of Tolkein's poem, "Namarie". Also, the following passage of the song refers to Mordor, Gollum, and "the evil one".
"Mine's a tale that can't be told, / My freedom I hold dear; / How years ago in days of old / When magic filled the air, / T'was in the darkest depths of Mordor / I met a girl so fair. / But Gollum, and the evil one crept up / And slipped away with her." This is believed to refer to Frodo Baggins and the One Ring in his quest to get into Mordor.
In "Bron-Y-Aur Stomp", the blue-eyed dog is named Strider, after Aragorn, who is occasionally called Strider in "The Lord of the Rings" books.
https://en.wikipedia.org/wiki/Bron-Y-Aur_Stomp
https://en.wikipedia.org/wiki/Ramble_On

sorry that I couldn't fit it on one reply

In "Stairway to Heaven", the lines "When I look to the west" (spoken of a lot in the books; Valinor, they leave to go there at the end) and "All that glitters is gold," are also obvious LOTR references.

https://en.wikipedia.org/wiki/All_that_is_gold_does_not_glitter

I forgot to add "My spirit is crying for leaving" after "When I look to the west", your spirits go to Valinor (in the west) after you die in LOTR and it also could be referring to Frodo who wanted to go there ("My spirit is crying for leaving") because of the wound he got from that blade he got stabbed by that couldn't heal, and later does go there.