To unlock the realm of Niflheim in you God of War must find 4 encrypted chests scattered in the various game locations. They are purple and have a "face" padlock. Niflheim is an optional Realm consisting of a series of random labyrinths. The best armor in the game can be found there, as well as the latest update for the axe. Getting the ciphers is also important to unlock the Trilingual trophy.
Below are the 4 easiest to find Niflheim ciphers. None of them can be missed unless there are big oversights and they can be collected in any order (still after the end credits).
# 1 - The Mountain
This chest is located on the Mountain Summit, the summit of the mountain, (you can access it from the quick travel gate called "Summit"). It is in a hidden side area. Go to where the Portal is (from the tree where you first met Mimir), cross the mountain pass into the mountain. On the left side of the route there is a small part where you can press "circle" to go down. It is not difficult to see where to go down, that part of the mountain pass is slightly destroyed (it is in the outside area, not inside the mountain). After doing this, the chest will be in front of you.
2 - Watchtower
The Watchtower is located in the region on the right side of Lake of Nine (east). It is a small area, the purple trunk is on the right side of the beach where you can dock your boat. It's behind a pointed barricade. To make the barricade disappear, hit the sign on the left side of the island.
# 3 - Ruins of the Ancient
The Ruins of the Ancient are in the upper left corner of the Sea of Nine. There is only an enemy there and a purple chest, very easy to spot. It's one of those islands you would never go to look for the chest, because there's no real reason to ever go there during history. But since there's one of the easiest ciphers to get, now you have a reason to go there.
# 4 - Coucil of Valkyries
The region of the Valkyrie Council is located north of Lake Nine. In front of the two large statues overlooking the Mason Canal, there is a small beach on the right side. Anchor the boat there, walk along the beach and you will notice two explosive red crystals. Ask Atreus to throw electric arrows at them to detonate the rubble on the path. Then you can reach the purple crate around the corner.
There are more than four encrypted chests, but these are the easiest to get. After obtaining all four, go to the Realm Travel Room and select Niflheim as your destination. Remember that for the "Trilingual" trophy you need to hunt for these collectible items.