Dagor Bragollach

(Battle of the Sudden Flame) As supposed by Devamitra this song's lyrics (as long as "Ungolianth" lyrics) are from Silmarillion directly. He says, that he can hear

....
Streams of fire flow down Thangorodrim
....

in the song. Here's the part of Silmarillion about the Dagor Bragollach. This part is one of the most (...er beautiful is not the word in this case) in the book.