I'm trying to remember the title and author of a novella I read some time after 1990. I believe I read it in an anthology and I also think it would have been a current or near-current work.
It was written in an artificially archaic style / voice. I believe that it may have been intended to invoke a much earlier (Victorian era?) work, and/or was set in that work's "universe".
It is set millions of years in the future, and the remnants of humanity live in a sort of skyscraper / fortress, surrounded by monstrous, shadowy creatures that are closing in on the fortress at an incredibly slow – glacial, geologic – pace. The rest of Earth has already been destroyed, and it is assumed that humanity will finally be wiped out at some distant date when these visible – but very, very slow – foes finally arrive.
The city is powered by geothermal sources and the plot revolves around a new geothermal source – more distant from the inexorable (but slow) foes – being discovered by the protagonist. This buys humanity some millions of more years of life. I think this happens as the result of events kicked off by a love triangle.
Editing in a couple more story details:
I remember an aside about how long ago, before the fall of Earth, men had traveled to other planets and stars, but discovered that the dark forces had already destroyed those planets and consumed all of their life in the distant past. This exposition may have been conveyed by the protagonist viewing a pictographic or hieroglyphic memorial or history of the events.
During the protagonist's expedition outside the wall of the fortress, when he is about to be consumed by despair, for a brief moment the thick clouds that have choked the skies for eons part just enough for a single star to become visible, and this inspires the protagonist to continue on.