After Zelda in the castle is revealed to be an impostor, you can help investigate the Ring Ruins in Kakariko Village.
Investigate the Ruins
After you start the quest (see above), use Ascend to enter the floating piece of the Ring Ruins in Kakariko Village, and then take a picture of the stone slab. Go down and talk to Tauro to have it translated.
Go to the Tower
You can meet up with Tauro and Calip at the Popla Foothills Skyview Tower.
Talk to Calip
Talk to Calip at the base camp at the tower to find out that Tauro went ahead to Dracozu Lake to the southeast.
Meet Up with Tauro
There is smoke from Tauro's campfire at Dracozu Lake, which can help you find it. Also, you can use the tower to skydive to Jiukoum Shrine in the hills north of Dracozu Lake. Then glide down to the lake.
Read the book at Tauro's camp to find out that he has gone inside the ruins.
Enter the Ruins
If you look east from Tauro's camp site, you see an open door. Before you go there, you can optionally go north and check on the glowing Goddess Statue to start the side quest Goddess Statue of Courage.
Then, from Tauro's camp site, go east and through the door. Talk to Tauro and there will be a cutscene.
Afterward, open the chest at the top of the stairs to get the Charged Shirt.
Tauro says that the Sensor+ feature of the Purah Pad could help find the other treasure chests, so you might want to unlock that if you haven't already.
The treasures will be found along Dracozu River, which is the "wide-mouthed forest serpent" mentioned in the mural.
There is a door at 0983, -2690, 0007. Destroy the boulders blocking the entrance, then open the chest to get the Charged Trousers.
There is another door at 0967, -2797, 0007. Burn or cut the vines blocking it, then go in and open the chest to get the Charged Headdress.
The mural said to offer a Zonai Charge at the tail, so go west along the river. You will find a door at 0690, -2745, 0014. Go to the back of the room, make sure you have equipped all of the pieces of the Charged armor set, and drop a Zonai Charge on the altar.
There will be a cutscene, revealing a structure that was previously hidden by clouds.
You can use the Popla Foothills Skyview Tower to reach the Thunderhead Isles.