Horse Guai Quest: How to Unlock the Dark Thunder Transformation

First Meeting (Chapter 1): Travel to the Back Hills Shrine in Bamboo Grove. Head into the cave to the left at the bottom of the path and exhaust the Horse Guai's dialogue. He will move on after your next rest.

Second Meeting (Chapter 2): In Sandgate Village, start from the Village Entrance Shrine. Cross the bridge, turn right past a small shrine-like building, and head left towards an enemy with a shield. Follow the building around to the left and up a small slope. Continue into the village and find the Horse Guai tied to the front right pillar of a large shrine structure. Exhaust his dialogue and rest.

Third Meeting (Chapter 3): Progress through Chapter 3 until you reach the New Thunderclap Temple. From the Temple Entrance Shrine, go through the temple doors, past the left side of the statue, and exit outside. Head up the stairs, then down stairs to the left into a building. Go up the stairs inside, exit the building at the top, turn left, and check behind the fifth wooden pillar on the left to find the Horse Guai. Exhaust his dialogue and rest.

Fourth Meeting (Chapter 4): Reach the Middle Hollow Shrine in the Webbed Hollow. Turn right from the Shrine and follow the path to a torch on your left. Drop down platforms into an open area to fight the Centipede Guai. After defeating it, follow the path at the back of the area, down a slope, and take a left at the bottom. Attack the yellow cocoon at the end to release the Horse Guai. Exhaust his dialogue and rest.

Defeat Keeper of the Flaming Mountains (Chapter 5): In Furnace Valley, start from the Valley Entrance Shrine. Continue past the two large pillars, down into the magma area, and onwards until you ascend again. Follow the path to the Cloudy Mist, Misty Cloud boss fight. Defeat this boss, then proceed to fight the Keeper of the Flaming Mountains. If you missed the Horse Guai in Chapter 4, this is your last chance to interact with him before failing the quest. After defeating the Keeper, return to the Valley Entrance Shrine.

Final Meeting (Chapter 5): From the Valley Entrance Shrine, follow the path forward, turn left, then right after a pillar, and descend into the magma. Continue right through the area until you find a statue where the Horse Guai will appear one last time. You must fight and defeat him.

Acquire Transformation: After defeating the Horse Guai in the final meeting, inspect the statue to find and interact with a whip. Pull forcefully to acquire the whip and unlock the Dark Thunder Transformation.