It's a fantasy novel which I picked up second-hand (and thus don't have a precise publication date) somewhere between 1992 and 1995.
Based on the condition and cover style, I'd say it was published in the early 90s.
It was the first or second book of a trilogy, probably second, with a vaguely Celtic background, but set in an entirely imaginary world - i.e., not historical or semi-historical fantasy.
The language used was very formal, definitely "epic" fantasy, and serious in tone, with no humour whatsoever.
The main character was presented as a man who was the pre-eminent warrior of the human race. He did not have a name, but rather a title of "the XXXX", where XXXX was (I believe) a Celtic-flavoured word.
The world was undergoing an invasion from a non-human army that gave no mercy whatsoever - a war of extermination (this was, I'm fairly sure, a major plot point; no quarter was given by either side).
The "leader" of the invasion was a demon or god that was from outside the world; it had not actually entered the world yet, but was approaching it. I have a memory of its name being "The Beast" or "The Beast from Beyond the World" or something similar.
Scenes I remember:
- the climax of the book, where a major female character was murdered at her wedding by a demon in insectile form (like a large fly), which was thereupon pursued by the main character. However, although he caught up to the demon, he was trapped by another demon, chained and captured.
- the epilogue showed the major villain approaching the world.
A major plot point was that the enemy was systematically exterminating female leaders/warriors, as a campaign to weaken the human forces.
I am fairly sure the author was male.
It is not:
- the Shannara series
- the Drenai series
- a major fantasy series that any fantasy fan would be familiar with
Any help would be greatly appreciated. It has bothered me for more than ten years - I've spent hours on Amazon trawling through early-90s fantasy novels trying to track it down.