Not sure how he got there, but I just discovered Magnar meditating at the top of a mountain...
And here's the savegame...
https://dl.dropboxusercontent.com/u/89188843/LH_1.1-bug-mountain.EleSav
[UPDATE: in the end, this meant I could only do magic or quest victories; I couldn't make diplomatic contact with Magnar, so no alliance victory, and I couldn't fight him or target him with spells, so no conquest victory. I discovered this after wiping everyone else off the map, and then being confused about why I hadn't won...]
