God Of War: 7 Optional Bosses & Challenges You Must Find
5. Muspelheim's Impossible Trials
You can actually access Muspelheim around the midway point in the game after acquiring all three pieces of the rune required to activate its Bifrost portal, but I wouldn't recommend it.
Instead, save your strength until after the main campaign. There are six specific arenas and one final fight against another Valkyrie to get through, but even after that, the "Impossible Trials" open up.
Excruciatingly hard variations on what you've been doing to unlock them, you need "only" clock any three of the six to trigger another final round where wracking up as big a body count as possible drops ultra-rare crafting components.
The Impossible Trials themselves involve everything from not getting hit to defeating enemies in a specific circle on the ground. They're by far the hardest thing in the game, and you'll need luck as much as skill to survive.