I think he is climbing the montain ?
He has idle animation. Doesn’t try to walk.
After leaving/returning the map a few time, mob came down and was fightable.
Ah, thank you @seewen for the report! Looks like the navigation system in this room may be a little too generous, or a spawn may be too close to other geometry. I’ll take a look at this room and get it patched up. Thank you for the screenshots!