Pristine Sanctum is a location in Tears of the Kingdom.
Tears of the Kingdom
The Pristine Sanctum is a hidden chamber, found behind the waterfall of Mikau Lake. Link can access it by simply swimming through the waterfall, or by dropping down the hole from up above at Lulu Lake. King Dorephan and Muzu can be found there, after the King was injured due to a battle with a Sludge Like that was summoned by Princess Zelda.
As part of the Sidon of the Zora and Clues to the Sky quests, Link will travel to the Pristine Sanctum to meet with the King. Here the King will give him five King's Scales, which Link will need to use when at the Floating Scales Island.
After the completion of the Water Temple, Dorephan and Muzu leave the Sanctum, which is thereafter uninhabited.
A Bubbulfrog hides on the cave wall behind the throne, to the left (as viewed from the front of the throne).