To unlock the Muspelheim Realm in God of War you must find 4 encrypted chests that are scattered in the various game locations. In Norse mythology, Muspelheim is the Kingdom of Fire. These cipher chests are purple and have a "face" lock. Muspelheim is an optional Realm that consists of some challenges that can be played on different difficulties. Unlocking the Kingdom of Muspelheim is important in the end of the game to get the best possible equipment. Also, by unlocking the Kingdom of Niflheim you get the Trilingual trophy.
Below are the four simplest Muspelheim ciphers. They can be collected in any order (even after the end credits):
# 1 - The River Pass (Witch's Cave)
The first is in the Witch's Cave in the River Pass region. It's a region in the south of the map, the area near the turtle house. Go to the gate for the quick trip "Witch's Cave Lower Level". It is unlocked automatically during the story. Take the elevator to the Witch's Cave. Behind the elevator is red tree sap. You must have Atreus shoot it with electric arrows (even those unlock during the story about halfway through the game). Behind the red tree is the trunk. Easy to find if you have the arrows already unlocked.
# 2 - The Mountain
This is on a route that you will have to visit in order to move forward in the main story, towards the top of the mountain, near the portal at the top. There is also the tree where you meet Mimir. It is in beautiful view on the path of history. Go to the gate for the quick trip called "Summit" and it will be on the road on the left.
# 3 - Cliffs of the Raven
The Cliffs of the Raven region is located on the right (east) side of Lake Nine. Go to the end of the area (it is very small). There you are attacked by some dark elves. Kill them. Then you can see a wooden barricade on the wall with a golden chain next to it. You can't reach the chain from below. So what you have to do is destroy the wooden barricade on the wall and press "circle" to lift Atreus up there. He will climb through the hole in the wall behind the barricade, and then lower the chain for you. Climb up and the Muspelheim cipher will be right there.
# 4 - Forgotten Caverns
It is located at the highest level of the Forgotten Caves. This region is located on the left (west) side of Lake Nine. You must approach from the docking point to the south, where there is a Mystic Gateway. The north pier leads to a different area. Once you reach the highest level of the caves, there you will find the chest in plain sight.
There are more, but four are enough and these are the easiest to get. After getting all 4 encrypted chests, go to the Realm Travel Room and select Muspelheim as your destination. Remember that for the "Trilingual" trophy you need to find these collectables.