Originally done for the cover of The Book of Lost Tales 2, Grafton Books, 1992

Source

More info on the subject matter https://lotr.fandom.com/wiki/Door_of_Night

But Morgoth himself the Valar thrust through the Door of Night beyond the Walls of the World, into the Timeless Void; and a guard is set forever on those walls, and Eärendil keeps watch upon the ramparts of the sky.