Bah, it's been said. That'll teach me to post on a whim.
I still think it's a safe assumption that The Legion as we experience it in TFL is an elite army. Alric leads them, true- but it took the Legion to rescue him in the first place (assuming the heroes are somehow attached to the Legion, which I'd assume they were).
The Legion is either elite, or incredibly lucky/unlucky. They find the codex, are trapped inside the tain with Murgen and Cu Roi, kill The Watcher, rescue Alric, kill Balor etc. I'd say their presence was not mere coincidence. Nor was it by default. We know there were other forces in the world.
I *think* the Legion becomes trapped east of the Cloudspine. Or are they just the only force to advance that far?
LR