I'm pretty sure that this is one of the books of the Runesword (1990-1992) series, by various authors. The mixed bag of adventurers, Elizabeth the witch, Endril the elf, Hathor the vegetarian troll and Caltus the warrior are recruited by the god Vili to retrieve the titular swords in order to fight The Dark Lord of Mistwall.
Bith, then Hathor, then Cal in the rear, joined him on the flinty crest. They drew a sharp breath at the strange sight. On this cloudless day in this cloudless clime, far to the west. they beheld the common enemy of all the speaking races.
The thing was a line of cloud, or fog: a storm cloud come to ground. The bank was tall, very tall, reaching almost to where normal clouds would fly. The clouds that formed this bank were not still. They roiled and boiled and fluttered and rippled up and down its sheer face. Color flitted through the cloud like a dirty rainbow—grey where it touched the ground, a tinge of purple here, a jot of red there, green where the sun hit it along the top, with orange streaks. But none of the colors were beautiful. They were the colors of bruises on skin, of mold on vegetables, of dried blood or vomit, of sun-bleached bones and things long dead.
But the most curious feature of the cloud was its straight-ness. True, the wall of cloud billowed out here or receded there, but its front edge was as cleanly cut as if with a knife.
The tremendous wall ran from north by east to south by west, and it stretched from horizon to horizon and obviously beyond. It was all the eye could see in looking west, a giant sea wave poised to come crashing down on all civilization.
Cal said aloud to no one in particular, "So that's it. eh?"
"Aye," replied Endril. "The Mistwall."
The party visit a Castle Cairngorm in the first book, and the name crops up in later books. But despite my quote above (from the first book), I think it must be the second book Skryling's Blade, by Rose Estes and Tom Wham, you remember since in that book Elizabeth and Endril are captured.
