With the rain finally cleared up, we can look to finish off this questline and be done with Sacred Stone Puzzles for good. To get access to Part 4 of this quest, you must complete Part 1, Part 2, and Part 3. With those completed, you will be directed to speak to Kaji, who is in the ruins of Fort Mumei, which is to the west of Fort Fujitou. The fastest way to get there is to glide from the westernmost teleporter near Fort Fujitou. Be sure to take the small detour to unlock the Waverider, as it will help immensely in this last portion of puzzling.
Once you speak to Kaji, you will begin Part 5, the fifth and final quest in the questline. To complete Part 5, we must solve the last Sacred Stone Puzzle, which starts at the Sacred Stone Mound near the Waverider Waypoint.
This Sacred Stone puzzle is perhaps the most straightforward in directions, as we are essentially trying to fire the Electro shot in a circle (or hexagon?), with it bouncing clockwise. To start this, aim the Sacred Stone Mound to the east, at the very crooked Thunderbearer Mirror.
Next, angle the crooked mirror upwards and rotate it to face the southwest, continuing the circular motion we are aiming for.
Then, you have to aim the next mirror downwards and rotate it to the southeast so that it will just barely avoid hitting the ruins and keep the Electro Shot going to the next mirror in the circle pattern.
Now at this incredibly crooked Thunderbearer Mirror, you must angle it downwards and then rotate it to face roughly to the east at the upright Thunderbearer mirror that is next in line in this circle.
After that, continue to the next mirror and rotate so that it faces the northeast. This will point it at the last Thunderbearer mirror.
Lastly, point this final mirror roughly in the western direction. This should aim it directly at the Ward in the center. The next time you strike the Sacred Stone Mound, it should trigger the cutscene. Once you break the barrier around the Ward, an unpleasant surprise awaits you. You will have to fight a Maguu Kenki while constantly effected with Hydro, as the battlefield is flooded.
Once you defeat him, you will be able to find the Warding Stone just to the west of the waterlogged platform, and you will have to climb up the tallest ruin (seen below) to glide down on top of the Ward itself to collect the Pearl.
All that’s left is to submit the items and talk one final time to Kaji, and the questline is completely done!