Yachimatahiko’s Trial is one of the challenges in the Three Great Martial Trials World Quest. There are multiple parts to this World Quest and Yachimatahiko’s Trial is the first part. Here is how you can solve Yachimatahiko’s Trial.
After talking to Eboshi and following the quest area, you should be teleported to Yachimatahiko’s Locus Island. Talk to Uda here and unlock the Teleport Waypoint as well.
Glide across the gaps and you should reach the first quest area, locked out with a gate.
Instead of hitting the triangle statue on the lower level, try using characters like Keqing or the Geo Traveler to get to the upper level. You will see a triangle statue emanating a blue light on the southern tower.
Hit it once and you should be able to get access to the flame across the triangle statue. Touch it and go to the left area and you will see a mechanism that can change Evernight to Whitenight and vice versa. Switch to Whitenight and glide down the area.
Release the second flame on the room to your left. Switch to Evernight again and go towards the uppermost quest area.
You will see that the flame is floating in the air. Go to your right and you will see a huge tree bark and some ruins. Climb it and glide down towards the flame to release it.
Lastly, change to Whitenight and go to the quest area. Hit the triangle statue once and change back to Evernight. Hit the emanating blue triangle statue next to it once to solve the puzzle.
Touch the flame to complete the trial. Go to the eastern part of the island and talk to Uda.
A cutscene will occur and that completes Yachimatahiko’s Trial!