Would I be able to use the line "elen síla lúmenn'' omentielvo" quoted in the book The Lord of the Rings in a song without breaking any copyright laws?

Without giving an exhaustive discourse on the concept of "fair use," the Copyright Act does not preclude the use and reproduction of very small extracts from a much larger entire work. This result may be vary depending on other factors. If there are significant commercial aspects to your proposed use, you should consider obtaining permission from the copyright owner for your proposed use. Such permissions are frequently given without charge.