Any books where the sentient species face off against an implacable horde like the Zerg, Tyranids, The Vord, or The Flood?

Basically as the title says. I'm in the mood for hopeless struggles against an apocalyptic threat.

One that scratched this itch was the Codex Alera series with the Vord. On the sci-fi side, Reality Dysfunction by Peter F. Hamilton also has a lot of overlap with this (and I love that trilogy) but I've already read that one.

The Demon Cycle kind of sort of has this, but aside from the fact that the series goes off the rails, that one is less of an active invasion and more just a series of walled off cities trying to survive in a mostly stable status quo until humanity is able to start fighting back, so not quite what I'm looking for.