On An Skellig, in Skellige, talk to two women in the town square of Urialla Harbor. The women will only appear when the storm is over -- so early on when you visit Skellige or after you've completed The Tower Outta Nowheres (Level 30, but doable at level 27) later in the game. They will give you the Level 16 quest, The Path of Warriors.

Follow your objective marker to the Trail to Yngvar's Fang location. Learn about the trial from Gunnar and be on your way. Careful, though -- the path has gaps and deadly cliffs. Don't be overzealous when fighting harpies and fall to your death. Keep climbing and jumping your way to the top.

You'll get to a castle ruin with a door. You can take a little detour and explore to find Bear School Witcher Gear, if you haven't done so. Keep following the "steps" going up outside until you get to the top of Mount Aardeklove ("clump of Earth").

Follow your quest marker to now find the cave entrance. You can backtrack or find another way down, up to you (just save your game before you go sliding!).

As you approach the cave, now is a good time to use Cat and Killer Whale. You'll have to go diving. Dive and swim forward, using your mini-map to guide you. Look for an area with roots hanging down; you can catch a breath here. There's also a treasure chest below straight ahead with some boots and goodies for you.

Right at the end, the cave becomes shallower. If you use Witcher vision, you'll see a submerged cabinet. Loot it to get the Path of the Warrior Completion Token; the proof you need.

Climb out of the water and head upwards. Watch your step. You have to jump the gaps to the ledges marked with white paint. You'll emerge at a tiny band camp (not an actual marker).

Return to Gunnar, who rewards you with fame and glory (EP) and a diagram and armor.