Go through the door to Surt's realm. Follow the path and approach the waterfall - you will find there the face of the mother. There will be a couple more places in the game, where the face will be hidden. Most of the time it blends in with the environment and takes some effort to be spotted.
Get across the bridge and keep following the path. After a while, enemies will appear. You know what to do with them. Try paying attention to the voices in the heroine's head - they will warn you if somebody's behind you.
After the fight, approach the door and you will get two runes. Go left, get up the hill on the left and activate focused vision. Use the two wooden poles to make the first rune (H-shaped).
Then get down and approach the crucified victims. This time, you have to look at a proper angle: the tree should be in line with one victim, so that it produces a shape like the one above. Activate your vision and snatch another symbol. Go to the door and get on the other side.