EDIT: I remember, and miss, Grim Portents zine. This was issue 1 of 2.
I’m having a mental blank. Was there a zine or supplement years back which had an adventure in arctic mountains and ice-caves, where the ice-cave had a mirror-self who would come out of the lake to replace you? There were also bear-riders mentioned outside the adventure but my mind had conflated everything because of the theme.