We can discover the masked men are
Merregons. The large hellbeast I'm not sure about. I do wonder whether we might get more of this story in the full release, particularly as it looks as though there's a part of Grymforge not yet accessible, ie the temple Nere was trying to get to when trapped by the rockfall.
I'm not sure whether you'd tried this fight with Nere pre patch 9, but I never had any issues with various NPCs going hostile before that release, though I think a few folk did report the odd problem. It does look as though something has changed in the most recent patch that has made this showdown in Grymforge far more frustrating, and I assume that's going to be fixed again by full release if not before.
