Apart from the regular moves that players perform in the game, Kratos can also use Light and Heavy Runic Attacks in God of War Ragnarok.
These don't consume mana but come with a considerable cooldown time.All three weapons that Kratos uses in God of War Ragnarok have Heavy and Light Runic Attacks associated with them.
The Njord's Tempest is a move that falls under the second category and is for the Leviathan Axe. The Rune for this attack can be found in Althjof's Rig.
Here's a quick guide on how to collect this item in the game.A Legendary Chest containing this Rune can be collected after destroying the second mining rig in Althjolf's Rig.Read more on sportskeeda.com