I borrowed this "Best Scifi short stories of [year]". One of of them said it was based on/influenced by another actual book or book series. There was a particular name for the cannon/universe it was based on, I just can't remember what it was and I don't have that book the short story was in anymore.
Way forward in the future, so far it's even past the twilight years, when mankind is burning out in a dark and inhospitable land where the only light is burning on what little fuel is left. They bunkered up in giant pyramid fortresses of essentially adamantium masterwork scattered about the land, but even those are ancient history. All the other forts the character comes across have been all destroyed by various untold horrors and lay in varying degrees of ruins. No one (or maybe it was very few) even remember there being other fortresses let alone refuge outside the walls. As he travels, he scurries low and tries to make himself small and invisible to hide from the incomprehensibly surreal horrors and monsters of all conceivable and inconceivable shapes and sizes lurking and waiting out in the murky gloom to preform horror that is implied to be > lovecraftian. While he's gone, what will probably be the last surviving generation of man clings to the dwindling supplies that are left as they have they all have their backs into a corner and the darkness presses in on all fronts.
There was the "word" which the monsters could not speak or understand, and it serves as the only way to catch doppelgangers. I can't really remember any other specific details.