The Night Land

The sun has died, as have the stars. Not a solitary light shines in the heavens. The days of light are nothing by a legend -- they are a story told to soothe children. The last millions of humans still live in their Last Redoubt -- but the end of their days is at hand.
First published in 1912, as a work of fantasy it belongs to the Dying Earth subgenre.
The importance of The Night Land was recognized by its later revival in paperback by Ballantine Books, which republished the work in two parts as the 49th and 50th volumes of its celebrated Ballantine Adult Fantasy series in July 1972.
Of The Night Land Clark Ashton Smith wrote "In all literature, there are few works so sheerly remarkable, so purely creative, as The Night Land. Whatever faults this book may possess, however inordinate its length may seem, it impresses the reader as being the ultimate saga of a perishing cosmos, the last epic of a world beleaguered by eternal night and by the unvisageable spawn of darkness. Only a great poet could have conceived and written this story; and it is perhaps not illegitimate to wonder how much of actual prophecy may have been mingled with the poesy." Lovecraft and Smith weren't wrong: this is, perhaps, the greatest single work of fantastic fiction in the English language