###Mountain Interior puzzle solutions and how to reach The Summit
Before you reach the summit of the mountain, you will have to contend with some icy foes and open the final few chests in the uppermost chambers of the Mountain Interior. These chambers also feature another puzzle involving a Nornir rune chest.
After destroying the first wall of red sap guarded by Sindri, go inside and immediately turn right when you have the chance. This will lead you to a legendary chest that can be opened by using a Shock Arrow to break through the red crystal that sits atop it.
As you make your way to the chamber that is located at the end of the short tunnel, be on the lookout for a new kind of foe, a Nightmare that is capable of possessing Reavers.
Take care of it by stunning the possessed Hel-Reaver, which will remove the Nightmare from it. After that, concentrate on the Nightmare in the usual way (using ranged attacks), and only then take care of the regular Hel-Reaver. To proceed further up the mountain, you will now have to solve a puzzle.
To begin, face right and aim your gun at the crystal embedded in the nearby red sap, which should be at head level.
In the smaller chamber that can be found behind it, the third Cup Artefact that is part of the Bottoms Up set can be found.
The next step is to pull on the wheel that is located close to the entrance. This will cause the bucket containing the now-freed red crystal to rise to the surface. At the point where it is adjacent to the higher red sap just to the left of the bucket, bring up the Pause menu and have Atreus fire another Shock Arrow at it. This will cause the sap to the left of it to explode and cause the rocks to fall, giving you the opportunity to use the rocks as a new platform and proceed.
Continue forward while keeping an eye out for the new nightmares that will soon appear.
As you proceed down the hallway, you will come across additional Nightmares and Hel-Reavers. In addition, you will notice another wall of red sap on your left as you travel further; we will come back to this in a moment. For the time being, proceed forward into the next room, which is directly in front of you. There, you will face off against three additional Hel-Reavers and locate a Nornir rune chest on the south-eastern wall.
There is some red sap holding up a bridge above you in the opposite direction, in addition to the red sap you just went past in the tunnel, and of course, there is a little world tree stump just nearby with some red crystal that you can pull out and throw. All of these red saps can be found in the same area. You already know how to proceed!
First, seize the crystal, then head back down the tunnel to destroy the red sap, and finally, enter that room’s interior and climb up the right side of the room’s highest wall to find the first rune. In addition to that, there is a brand new Lore Shrine for you to peruse.
Now that you have the crystal, head back to the area where you found it and throw it at the sap that is on the bridge above you. This will cause it to fall, which will bring forth additional foes for you to eliminate.
After crossing to the other side, there is another Lore Marker on your right hidden behind more sap; you will need to return for some crystal and smash this in the same way as the previous one in order to read it with Atreus.
If you cross the bridge and immediately turn to face that direction, you will find the second rune at eye level directly to the north-west of the bridge.
The third is located on a higher level, and the only way to reach it is to pull the bridge you just crossed back up to where it began and use it as a ramp. This will allow you to access the higher level. You will need to pull the wheel to the right of the bridge, then you will need to freeze it in place by throwing your axe at the cog.
When you reach the very top, the third rune will be directly to your right.
You should also look to your left, where there is a coffin that holds a Hel-Reaver. Defeat it to earn the loot that you would have gotten from the coffin, then bring back your axe and jump down onto the bridge below, which has now been lowered, to return to the Nornir rune chest. Another Idunn Apple can be found within.
After crossing the bridge a second time, proceed straight ahead into another brief tunnel, and then climb the wooden panels that are clearly marked to reach a set of doors and, at long last, The Summit.
You’re getting very close now!
After repairing Atreus’ quiver using one of those unique arrows that he obtained from Sindri, press on forward through the deep snow and breathtaking scenery to engage in combat with additional Hel-Reavers.
At the far end, climb up the marked parts of the stone, and as you make your way all the way around to the right, listen to the conversation being had by those gods who are in opposition to each other up above. When you reach the top, you’ll find a man who seems to be of Scottish descent entombed inside of a tree. He goes by the name Mimir.
Even though Mimir is very intelligent, he has some discouraging information to share with you: this mountain is not the highest peak in any of the realms. The Giant’s Fingers can be found in Jotunheim, which is an entirely different realm! If you want to spread the ashes of Atreus’ mother at the highest peak, you will have to go there, and getting there won’t be simple at all.
After the cutscene has concluded, retrieve Mimir’s severed head (this will become clear later on in the cutscene…) your next stop will be at The Witch’s house, which is located further down the path. At least, “Inside the Mountain” is finally finished after all these years!