I remember reading this late 80's (possibly early 90's), it would have been published about that time, in English. I read it in Ireland.
I feel like it was post apocalyptic fantasy, or a future settled world that descends into low tech.
Two things I distinctly remember, while a large party journeys to find an ancient weapon they're lead for sometime through tunnels by "dwarves", I've quoted that because it was obvious these were a subgroup of humans who had taken to living underground and ate an hallucinogenic fungus.
I remember the end of the book some of the party is ascending some massive tree or entering a massive forest, when with deafening noise the entire area "wakes up" and begins to fly away, it turns out it was one incredibly huge creature.
This has stuck in my mind because the end was confusing and I couldn't figure out what actually happened, or why it was significant.
Unfortunately I can't remember any other plot points, or character names.
I think the cover of the book was mostly red, and may have shown a distant view of an alien forest (but I could be completely wrong about that).
I think primary weapons were bows and swords.
I think the "large party" was comprised of our adventurers and a party of the dominant king/emperors soldiers, and there was some tension between the groups.
It read like the first book in a series, and the second book would pick up from the cliffhanger ending of the first.
It wasn't Darwath, Daggerspell or Shannara.