Apparently there are 4 hidden landmark locations on Solstheim that can only be reached by climbing/jumping up the mountains. I had to search for some information on this as I've been going crazy trying make my way up this impossible to reach landmark. It was the one near Saering's Watch that caught my eye, and it's called Frykte Peak.
The hidden landmarks are:
- Frykte Peak (Above Saering's Watch)
- Mount Moesring (South of Moesring Pass)
- Mortrag Peak (South of Castle Karstaag Caverns)
- Hvitkald Peak (West of Abandoned Lodge)
This information was found at the Bethesda Forums, topic: The Peaks of Solstheim (Location spoilers)
Have you found the landmarks, or better yet, made it all the way up!