Walk around the mountain, there's a path. Follow it and you will see some mist monsters with Lv. ??. Run past them, there's a giant door. It's locked. Abuse the buggy climbing (almost as bad as Skyrim lol) to go OVER the cave behind that door (that part took me over an hour), then slide down the other side. A short climb later you can get to the point of power lol. To go back you need to enter the cave (no monsters in there) from the other side and fall down some deep hole into a lake. Just follow that and you'll stand outside the mountain again. You even get a cool sword from the cave.
It's funny, I
literally did this 5 minutes ago
I wonder how you get that key though, that quest could be really interesting. When you fall down into that lake in the cave there's a part with shallow water. In the water stands a cooking pot, surrounded by human limbs. No monsters though. I bet the enemy didn't spawn because I didn't have the quest when I entered the cave