To Unlock Muspelheim Realm in God of War you must find 4 Muspelheim Language Cipher Locations. They are in the purple cipher chests that have a “face” lock. Muspelheim is an optional realm that consists of a few trial challenges that can be played on different difficulties. The last upgrade for the blades of chaos is also obtained in Muspelheim by killing the Valkyrie of the region. It plays an important role in the endgame to get the best gear possible.
Below are the locations of the 4 easiest Muspelheim Ciphers. None of them are missable and they can be collected in any order (still after the story in free-roam):
#1 – The River Pass (Witch’s Cave) – 0:54
The first one is located in the Witch’s Cave of the River Pass Region. It’s a region in the south of the map, the area near the turtle house. Go to the fast travel gate “Witch’s Cave Lower Level”. It’s unlocked automatically during the story. Take the elevator up to the Witch’s cave. Behind the elevator is some red tree sap. You must have Atreus shoot it with electric arrows (those too are automatic story unlocks roughly halfway through the game). Behind the red tree sap is the chest. Easy to find if you have the arrows unlocked.
#2 – The Mountain – 0:55
This one is on the main story path at the mountain summit, next to the summit portal. By the tree where you meet Mimir. It’s in plain sight on the story path. Go to the “Summit” fast travel gate and it’ll be up the path on the left.
#3 – Cliffs of the Raven – 1:33
The Cliffs of the Raven Region is on the right side (east) of the Lake of Nine. Go to the end of the area (it’s very small). There you get attacked by some dark elves. Kill them. Then you can notice a wooden barricade on the wall with a golden chain next to it. You can’t reach the chain from below. So what you must do is destroy the wooden barricade on the wall and press to help lift Atreus up there. He will climb through the hole in the wall behind the barricade, then lowers the chain for you. Climb up and the Muspelheim Cipher Chest will be right there.
#4 – Forgotten Caverns – 2:46
At the top level of the Forgotten Caverns. This region is on the left side (west) of the Lake of Nine. You must approach from the southern dock location, where there’s a Mystic Gateway. The northern dock leads to a different area.
There are more than 4 but these are the easiest to get. After getting all 4 ciphers, go to the Realm Travel Room and select Muspelheim as your destination. Finding all language ciphers is also required for trophy “Trilingual”.