The Leviathan Axe can be upgraded to a maximum level of 6 by bringing frozen flames to Brok. You can get most of the frozen flames from bosses encountered during the main story. If you miss one, it will appear under the Lost Items section in the shop.

The final frozen flame is obtained by trading in the Chilling Mist of Niflheim. It's locked inside a chest in the central chamber of the Niflheim realm. You'll need to collect 5,000 mist echoes to open the chest.

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Upgrade the Blades of Chaos
Chaos flames can be used to upgrade Kratos' blades up to level 5. As with the frozen flames, all but one can be obtained during the main story. To fully upgrade the Blades of Chaos, defeat the Valkyrie in Muspelheim to earn the Raging Inferno of Muspelheim, then exchange it for the final chaos flame.

New Game +
After finishing the game, choose New Game + from the main menu screen to restart the story with all of your previously acquired moves, equipment, and currency. Additionally, you can craft new armor including the “Cod of War” suit from God of War II.

God of War Secret Ending
After finishing the main story, return to Kratos’ home and go to bed. You’ll then see a special preview of the next God of War game.

Unlock the Muspelheim Realm
Unlocking Muspelheim requires you to find four Muspelheim Language Ciphers. There are more than four such ciphers hidden throughout the game, but these are the easiest to find:

Muspelheim Language Cipher Locations Location Where to Find It
The River Pass

Enter the cave underneath the witch’s house and look for a chest covered in viscera.
The Mountain

After meeting Mimir, descend the stairs and pass the mystic gateway to find a chest. Cliffs Of The Raven Land on the beach southeast of the temple. Look on the left side of the cliff for a small opening covered by wood. Smash through it with your ax, and give Atreus a boost so he can drop a chain for you to climb.
Forgotten Caverns

Go into the caverns northwest of the temple and climb to the upper level.
Once you have four ciphers, you'll have access to Muspelheim from the Realm Travel Room.

Unlock the Niflheim Realm
Unlocking Niflheim requires four Niflheim Language Ciphers. The easiest ones to get can be found in the following locations:

Niflheim Language Cipher Locations Location Where to Find It
The Mountain

Go up the steps to the green garden area, then drop down to the snowy path. Look along the left side for another area you can drop down to where you’ll find a realm tear and a chest. Lookout Tower Search the beach south of the temple near the Odin statue's foot. Ruins Of The Ancient After gaining access to Konunsgard, travel northwest of the temple to find a Soul-Eater and a chest on the beach. Council Of Valkyries Dock on the beach just right of the giant statues holding oars. Break the red crystals to clear a path and climb the cliff wall to find a chest.
Niflheim will be selectable from the Realm Travel Room after you collect the ciphers.

Golden Talisman of Protection
The Golden Talisman of Protection is a helpful accessory that can be obtained early in the story. After gaining access to the temple in the Lake of Nine, travel northeast to Stone Falls. You'll have to battle some revenants and solve a puzzle to reach the chest containing the Golden Talisman of Protection.

Legendary Forbidden Grip of Ages
To get the most powerful pommel for the Leviathan Axe, you must scale Muspelheim Tower in the southern region of the Lake Of Nine. It's only accessible via the Lookout Tower boat dock once the water level has dropped far enough for you to reach it.

Once atop Muspelheim Tower, stand in the golden circle between the two braziers with burning fires.