A fantasy series of books from before 2000, in which an evil overlord created a wall of cloudy darkness which slowly engulfed the world.

He imprisoned a girl - the main character I suppose - in a tower, standard torturing by hunger, cold. She communicated with the other characters with the help of some birds or insects through the only window.

There was a location called Cairngorm (which pretty much muddled all my searches). Some rats (controlled by someone?) chew the playing bones of orcs in the caves below causing a meek one to always win - generating a distraction.

There might have been a sword involved in defeating the overlod. I distinctly remember there being a god or powerful character named Vili (as in Odin's brother, but I don't recall any other norse connections).