Yachimatahime's Trial is one of the challenges you need to face to complete the Erebos' Secret World Quest, which can be activated after unlocking the three hidden islands in Enkanomiya. Once you've completed the prerequisite, head to the pool in the Evernight Temple.
Going to Yachimatahime's Locus
The exact location is circled in the map below. Make sure that your daylight setting is set to Evernight.
Once you're there, jump into the school of Ghostfish in mid-air. You may need to use a Geo Construct, use an Anemo Skill, or jump from a nearby landform to reach them.
The Ghostfish will guide you to Yachimataime's Locus, which is the once-hidden island on the top right of Enkanomiya. You don't have to click anything as you're being transported.
Upon landing on Yachimatahime's Locus, you'll encounter the afterimage of Eki. Speak to him to activate the Trial to release all the Flames of the High Gate.
All Flame Locations
Approach the Enkanomiya Phase Gate within the vicinity, and it will guide you toward what looks like a wishing well. Interacting with it will show you a rough location of all four Flames. Alternatively, reference the image below to help you locate them.
All the Flames are located quite high up, so consider bringing Geo Constructs or Anemo characters to help you climb faster. Once you spot the Flames, interact with them to complete the objective of this Trial.
After all four Flames have been released, follow the quest marker back to the afterimage of Eki, who will determine that you've passed the Trial. You'll unlock the Wonders of the World Achievement called "The Eel in Winter Sought".
Complete another part of the overarching World Quest by following our guide on How to complete Yachimatahiko’s Trial in Enkanomiya in Genshin Impact.