You can find the Pristine Sanctum behind the waterfall at Mikau Lake.

Cave Location

The Pristine Sanctum is at 3623, 0585, 0172, which is behind the waterfall that falls into Mikau Lake. It has another entrance at 3706, 0611, 0357, in Lulu Lake, west of Ihen-a Shrine.

Explore the Cave

You will need the Zora Armor. Use it to go up the waterfall within the cave. If you have completed The Broken Slate, King Dorephan will give you five of his scales, and you can come back and talk to him here if you need more.

If you go behind the King's throne, and go down to the lower area, you can find this cave's Bubbulfrog on the north wall near a wooden treasure chest. Then you can use the Zora Armor to go up the waterfall, through the hole in the top of the cave, which leads to Lulu Lake.