I reported this on the steam forum but didn’t get the usual response. So just in case I’ll mention it here too.
This is the location of the third tome on Skittergate. When you grab the tome you have to backtrack a short distance down this narrow cleft in the rock before you can go through the gap in the fence. When we picked up the tome a blightstormer spawned and casted on us, we were unable to hit him because he was behind the watchtower and it blocked our shots. It looks like he either spawned on the cliff edge in the background or on the back side of the tower.