LotR -- Book 1 - Chapter 08 - Fog on the Barrow-Downs

[Morthoron]

Lost and forgotten be, darker than the darkness,
Where gates stand for ever shut, till the world is mended.

Perhaps this is a Tolkienesque allusion to the Door of Night, behind which Morgoth is shut until the end of days, the Dagor Dagorath, and the recreation of the world. Bombadil would know this because he was, of course, not of Middle-earth and had read a few revisions of Tolkien's early Silmarillion works whilst hanging out in the wilds of Oxfordshire:

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.