This claims to be "First in the Chronicles of the Shadow War". Although the blurb doesn't say so, it also follows on from the film Willow (which rated a 4 from me, so I probably wouldn't have bothered with the book if I'd realised. But on the other hand, it does have Chris Claremont as co-author...).
The story starts one year after the end of the film. Willow Ufgood is back on his farm, not yet reconciled to being a wizard. Sorsha renames him Thorn Drumheller, portent for what is to come [I presume -- it hadn't come by the time I gave up]. Ten years later, after a great cataclysm, Thorn has been wandering around looking at various remains. He gets to the last one, no different from the rest, has a few minor adventures, then goes off to see the Princess Elora Danan. Foul play is afoot, and he gets thrown in a dungeon.
It is well enough written I suppose, but it is dreadfuly slow, and I wasn't at all engaged by any of the characters or events, so I didn't last beyond the first third. I gave up at page 136.
[If it gets better after this, please let me know, and I might have another go. [18 March 1997: Well, someone has let me know: apparently it stops being 'mind-numbingly boring' and gets 'really good' just after the point I gave up!]]