You get it by opening chests in the randomized Niflheim Maze. Used to craft and upgrade armour, Pommels, and Talismans with the powers of cursed mist. The mist bears a memory of this time as well, and becomes tangible enough to collect. Also dropped by low-level enemies mostly in Midgard. Used to upgrade various armour sets, as well as the Talon Bow. Interact with it to retrieve this crafting material. Used in a wide variety of crafting applications.
For 100% game completion you may find the God of War Collectible Guide useful. You will find almost all materials when you play through the story, side quests, and go for 100% game completion (find all collectibles).